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By Deepu Dharmarajan
Posted 3 years ago

CH5 | CONTROL TABLES

Signalling

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SIGNALLING BOOK | CHAPTER 5

CONTROL TABLES 

CONTENTS 

1.INRODUCTION

2. IDENTIFICATION OF ROUTES 

3. MAIN ROUTES

4. SHUNT ROUTES

5. APPROACH LOCKING 

6.POINT CONTROLS 

7.ROUTE LOCKING 

8.OVERLAPS 

1.INRODUCTION

Control Tables express the conditions for the setting of routes, clearing of signals and the setting and locking of points and ground-frames.

They are a vital part of the specification of  any signalling  installation  (in  conjunction  with the signalling plan). 

They specify precisely the controls applicable to all signals, points and other signalling functions. They are the starting point for detailed design and the  main reference documents for testing.

Control tables may be presented in a variety of different formats. 

Although a railway administration may at one time standardise  on a particular  format,  requirements  change over a period of time. 

It is quite normal to find several different  formats  in current  use (although for different installations). 

Although control  tables  are essentially  written  documents,  they are frequently produced as drawings on a large page containing several routes or points one above the other. 

Alternatively there are many advantages  in  using  a small  page  size with only one route or one set of points per page.

This section of the course will often show several  sets  of  controls  on  a  page  within  the notes to assist understanding and comparison. For practice  work, separate pages for each function will be used.

Two main types of control tables will be used. The first giving the conditions for the operation of signals and the second giving the conditions for operation of points.

There may also be a need for various other specialised types of control table,  e.g.  for  automatic signals, ground frames, level crossings etc. These will not generally be covered

here,  although  reference  may  be made  to some of  them  in other  section,s. 

Note :Detailed explanations  are made with respect to the  real track layout below (Bakaburke Junction -Figure 1 ) 

LHCT_Left Hand Caution Turnout

RHMT-Right Hand Medium Turnout

Table 1 -Route Table for Signal 24 

Details of  entries will be at the bottom of this article.

2. IDENTIFICATION OF ROUTES 

2.1    Types of Route

Each route may be one of two types:-

a)    Main Routes (M)

A main route extends from one running signal to the next running signal or to the buffer stops at a terminal station. 

A route Leading up to another running signal will include a further margin beyond that signal, known as the overlap.

b)    Shunt Routes (S)

A shunt route is one which allows for a shunting or other low speed movement which  is not already provided for by running signals, or where the route may possibly be occupied by another train.

 This includes entry to and exit from sidings, and wrong-line movements.

Where passenger trains are required to enter occupied sections of track, some railway administrations classify such routes separately as calling-on routes. TfNSW (Transport for New South Wales )  does not make such a distinction.

2.2    Naming of Routes

Routes are named after the signal from which they apply (the entrance  or start signal for the route). If a signal has more than one route, each must be uniquely identified. 

This will normally be by a letter or number suffix. In these notes, letters will be used with the left-band route labelled A and the remainder consecutively B,C,D etc. in geographical order, according to destination. 

Alternative routes to the same destination are identified by adding -1, -2, etc. to the route letter. The exit (or finish) of the route will be shown on the control table.    ·

If there is more than one type of route from a . signal, then the type is indicated by a distinguishing letter in brackets after the number, i.e. (M) or (S). These routes should already be defined on the signalling plan.

EXAMPLES (Refer with respect to Figure 1)

3. MAIN ROUTES

A main route proves that the section from one running signal to the next is clear  and  in addition proves that the overlap beyond is also clear. 

The  overlap  will  be  marked  on  the plan. Determination of the length of the overlap has already  been  dealt  with  in  the "Signalling a Layout" section. 

The overlap, and the controls associated with it will vary according to whether or not trainstops are fitted.

Signal controls are required at two distinct levels. The interlocking controls are those conditions which must be present to allow a route to be set. 

The aspect controls are those which permit the signal to show a proceed indication  after  the  route  has  been  set  and locked.


3.1    Setting of Points

Assuming a route setting system of  interlocking,  we must  list all  the points  which  will  be set and locked when the route is selected. 

These are interlocking controls. If any  of  the required points are locked in the position opposite  to that specified,  the route will  fail to set. 

To set a main route,the points must be lying in the correct position or free to move. The detection of the same points will generally appear in the aspect controls.

Trailing points in an overlap are also set, locked and detected in the same way as points in the route. However, facing points in the overlap which give a choice of possible overlaps are not usually locked. 

This allows the overlap to be "swung",  for example when setting the route on ahead from the exit signal.

3.2    Locking of Opposing Routes

The interlocking controls require that no opposing routes have been set. Where a signal has more than one type of route, the setting of one route will lock out any other to the same destination. 

All these routes will be listed under "routes normal" in the signal control table.

An opposing route may pass through all or any part of the route to be set, or its overlap.

3.3    Track Circuits

All track circuits in the route (and to the end of the overlap) are proved clear in the signal aspect controls. The first track in the route places the signal to danger and prevents it from reclearing automatically.

 It is designated as the Lever Stick track. 

Track circuits are NOT proved clear in the route setting controls, in order to allow the signalman to reoperate the route for a following train without having to wait for the first train to clear the section.

3.4    Displayed Aspect

All the aspect controls described so far will permit the signal to display  its most  restrictive main running aspect for that route (including Low speed  where  applicable).  

Clearance  to other less restrictive aspects will generally depend on the signal ahead, as  shown  in  the "further aspects" line of the signal controls.

Examples (Refer with respect to Figure 1 ) 

3.5    Foul Track Circuits

When a track circuit is not in the direct line of a signal route (or overlap), but one of its extremities is within the required clearance point for that route, then the track circuit is said to be foul of the route.

 It is unsafe to allow a train to proceed if any foul track circuit is occupied. Most foul track circuits can be included in the point controls to prevent the points aligning for the foul route. 

If  this is not possible, the foul track circuit should be proved clear in the signal aspect controls.

If  the foul track is conditional on the position of other points, it may be possible to set those points to the position which excludes the foul track circuit. 

This should only be done if it does not restrict the setting of other routes. The two ends of a crossover are an example of excluding a foul track by the setting of points.

Examples (Refer with respect to Figure 1 ) 

3.6    Flank Protection

Flank protection is the proving of additional track circuits and/or setting of points to protect  the route from irregular converging movements, i.e. to protect the flanks of the route.

Diversionary flank protection by points  not  in  the  route should  only  be provided  if  it can be achieved simply, and without inhibiting other valid routes.

The proof of track  circuits  clear  from  the line of  route  back  along  each  converging  route as far as the first protecting signal provides protection against overruns.

 It also increases the impact of track circuit failures and can be restrictive on some other legitimate moves. 

It also provides no protection at the time it is most needed  -  after  the train  has  passed  the signal.  An example would be the inclusion of 10A track in No. 8 signal aspect. 

This type of flank protection is not now generally favoured.

Setting and locking the points in the converging line to a position which will divert an unauthorised converging movement away from the legitimate route is simpler and more effective. 

This protection should be provided where it does not  restrict  other  permissible traffic movements.

The most important use of this protection is for the setting of trap (catch) points protecting siding or lines which are unsignalled or where vehicles may be stabled unattended.

Examples (Refer with respect to Figure 1 ) 

3.7    Junction Signalling

Where a purely speed signalling system is used junction signalling is handled  simply  by the  use of appropriate aspect corresponding  to the  required  reduction  in speed.  

The driver  will be given advance warning that he is to take a slower speed route by the display of  an appropriate aspect at the signal in rear of the  junction  signal. 

 Information  on  the  precise route is not essential.

TfNSW (Transport For New South Wales) signal aspects do not imply a specific speed but the usual practice is to display a medium caution at the signal in rear of a junction signal showing any turnout aspect. 

This would be a pulsating yellow for single light signals and green over yellow for double light signals. 

The junction signal itself will display a caution turnout aspect if the signal ahead (the next signal after the junction) is at stop or a medium turnout if the next signal ahead shows a caution or other less restrictive aspect.

Examples (Refer with respect to Figure 1 ) 

Other route signalling systems may not provide any indication of the turnout route at the previous signal. 

In this case, the application of approach control to the junction signal is used to regulate trains to the appropriate speed for the turnout route. 

The approach release may be either from stop or from caution, depending on·the turnout speed. There are two potential problems with this method of junction signalling.

a)    If the sighting of the junction signal is restricted, trains speed may  be  reduced  to a lower speed than the turnout speed.

b)    If  trains of widely differing speed and braking characteristics use the line, approach release from stop may be too restrictive to slower trains 
but approach release from caution may permit faster trains to take the turnout at excessive speed.

The BR system of junction signalling now employs additional aspects to give an advance indication of the junction at the signal in rear, especially where the line is used by trains with widely differing braking characteristics.

The junction signal will release to a less restrictive aspect when the train is within sighting distance of the junction indicator on the junction signal. 

This control may use an existing conveniently placed track circuit or, if no suitable joint exists, timed occupation of the track circuit. 

Additional tracks are not provided for the specific purpose of approach control -  the provision of a timer is considerably cheaper than a set of insulated joints. 

This also avoids drivers regularly anticipating the signal clearing when the train reaches a particular block-joint.

 It will also provide an earlier release for a slower train - this could  be of benefit to heavy freight trains where it may take several seconds for brakes to release.

3.8    Trainstops

TfNSW started rolling out ATP with ETCS levels ,Trainstops are decommisinoned ,but for the sake of learning its mentioned here 

Where trainstops are provided, the signal control tables must show their operation  in association with the signal. 

The trainstop associated with  a  main  signal  will  be  lowered when all aspect controls are present and proved down before the signal  shows  a proceed aspect. 

On replacement of the signal  to danger,  the trainstop  is proved  to have  returned  to the raised position before another route can be set up to the signal.

On reversible lines, trainstops within the route intended for the opposite direction of  traffic must be lowered (trainstop suppression) until the rear of the train has passed.

Controls must also be shown for intermediate trainstops, not  located  at  a  signal.  

The lowering of such trainstops will normally be permitted by the  signal  ahead  showing  a  proceed aspect or the timed occupation of the approach track circuit.

4. SHUNT ROUTES

Shunt routes are used for shunting and other slow-speed movements, e.g.  into  or  out  of sidings or for movements when the route ahead  is not  necessarily  clear.  

Any  route  to or from a shunt signal (including limit of Shunt signals) is classed  as  a shunt  route.  A shunt route from a signal which can also display main route is set with  an  alternative  entrance button on the panel.

A dwarf colour light or position light signal displaying a yellow aspect is given for shunt routes. Stencil route indicators are generally provided for shunt routes if there is more than one route from the signal. 

A route indicator is always provided for a wrong road movement. Where a route indicator is provided, it is not usually proved alight as all routes must be indicated.

Where a shunt route is set from a main signal, a subsidiary  yellow  light  is provided  below  the main signal aspect. This may also have a route indicator as above.

Shunt routes prove all points in the route, but do not necessarily prove  track  circuits  clear. The first track circuit in the route is normally included as  this  provides  the  lever  stick feature.

Shunt routes are normally provided with an overlap of 100 metres clear  of  other fouling moves. Unlike main routes, however, track circuits are not proved clear. 

Wherever possible, this overlap is achieved by setting and locking trailing points.

TfNSW  does not provide shunting or subsidiary signals with "last wheel" replacement. Signal replacement is always by occupation of the lever stick track(Birth Track) .

Examples (Refer with respect to Figure 1 ) 

When signalling trains into partially occupied station platforms, TfNSW  does  not require  the  train length to be measured before clearance of the signal.

Where a preset or facing shunt is situated in a main route reference should be made to this in both control tables. 

The main route should be shown as setting the shunt signal and proving it cleared while the shunt route should be shown as being preset by the main route.

5.    APPROACH LOCKING

Once a signal has been cleared, the route must not be released if the signal is restored to danger in front of the approaching train, unless certain safeguards are taken. 

This is enforced by applying Approach locking to the route.

In simple cases, a signal will become approach locked as soon as it has been cleared. 

However, in most cases this arrangement would cause unacceptable delay, 

so it is arranged that the interlocking looks back for an approaching train and the signal will only become approach locked when it is cleared AND a train is detected occupying an approach  track. 


If no train is detected approaching, the route will release immediately the signal is replaced to danger. This is called "comprehensive approach locking".

Once a signal has become approach locked, the route can only be released by either:

a)    The train entering the route (it then locks the remainder of the route by its presence.)

b)    A suitable time delay sufficient  to ensure  that  an  approaching  train  has either  come to a stand or entered the route (in which case  the remainder  of  the route will  be held by route locking).

5.1.    Release by Train Entering Route

The passage of a train into the route is proven by  the simultaneous  occupation  of  the two track circuits immediately beyond the signal in conjunction with proving of the power  off timer. 

The power off timer proving provides a delay in  operation  of  approach  locking releases following a power supply failure which would cause track circuit repeat  relays  to drop. 

This safeguards against a false release being given when power is restored.

Other systems often enhance this approach lock release by proving sequential operation  of track circuits usually the 1st and 2nd tracks  past  the signal  occupied  together  followed  by the clearing of the 1st track. 

This  is done  to ensure  that  a block  joint failure  between  the two tracks concerned, causing both tracks to drop, does not give a premature release of the approach locking. 

A further safeguard is often included by providing a stick feature on the approach tracks such that once the approach locking tracks have been occupied by the approaching train and the approach locking  has  become  effective,  then  the locking  cannot be released by clearance of the approach tracks.  

This  ensures  that  the approach  locking  is not released by a track circuit picking under a train due to bad rail conditions  or  poorly adjusted track circuits etc.

5.2.    Release After Time Delay

The delay applied before a route is freed if the train does not proceed through the route is intended to give time for the approaching train to be brought  under  control.  

For  a  main aspect the release time is generally 120 seconds  and  60  seconds  for  a  shunt  signal. However each case should  be considered  carefully  as in  some  cases  a  different  time  may be required. 

For example, 120 seconds for terminal platform starting signals may be unnecessarily restrictive and where the distance back to ' the previous signal is long  120 seconds may be insufficient to prove that the train has been brought under control.
 
The same approach locking is usually applied to all routes from a signal, so a subsidiary signal will have the same time delay as its associated main aspect. 

This is a  purely practical consideration to avoid the expense of providing more than one approach lock timer on any signal. 

This need not be the case with Computer Based lnterlockings as no separate hardware is necessary.

5.3.    Operation of Comprehensive Approach Locking

This becomes effective when the driver of an approaching train is likely to have  seen  a proceed aspect on the signal being replaced, or has seen aspects of previous signals which would lead him to expect to see a proceed aspect on that signal.

Comprehensive approach locking looks back at  the  approach  track  circuits.  For  shunt signals it is usually only necessary to look back at the berth  track circuit,  provided  it is at  least 200m  long.  However,  for  running  signals  it is necessary  to look  back  at least  as far as the sighting point of the earliest signal whose aspect is affected by the replacement. For signals with only a caution aspect in rear  this will  involve  looking  back  at  the section  in rear of the previous signal,  but  where  a signal  has  a medium  caution  and  caution  aspects in rear this will involve looking back at the sections in rear of the previous two signals.

Often these approach tracks will be conditional on one or both of the following factors:-

a)    A train being routed up to the replaced signal. This will depend on the lie of points. Facing points will exclude the approach if set for another route.

b)    The signals approaching the replaced signal displaying a proceed aspect.  If  the  previous signal is at red (and free of approach  locking)  then  it  is  not  necessary  to look back at the section in rear of it.

c)    The existence of the appropriate track circuits. Signals reading out of  non-track  circuited sidings will not have any approach track  circuits.  

These  signals  will  therefore be approach locked when cleared. As there is no means of detecting an approaching train it must be assumed, for safe operation, that a train is always approaching.

Route Normalisation

Route normalisation is provided at certain interlockings to release the route automatically. 

It operates once the train bas passed the signal and the approach locking bas released, provided that no other train is approaching the signal.

Such a facility reduces the signalman's workload by  removing  the  need  to  cancel  most routes after use. If any form of automatic  route  setting  is  introduced,  automatic normalisation must be provided on all associated routes.

As the normalisation  of  the  route will  release  the interlocking  on  other  routes  and  points, it is important that safeguards are provided against the momentary irregular operation  of  a track circuit releasing a route ahead of an approaching train.

It is therefore usual to use the same "look back" controls as the comprehensive approach locking to prove, in addition to the train passing the signal by operation of the  approach locking tracks, that no train is approaching the signal at the time normalisation is initiated.

This condition may need to be relaxed at terminal and other platforms where  trains  may divide, if Route Normalisation is required to operate for the first half of  the train  to depart. One option is to include additional sequential track circuit operation beyond the normal approach lock release tracks as an alternative to the "no train approaching" condition.

Where there is an automatic signal in rear, then the  "look  back"  controls  may  detect  a second train approaching behind the automatic signal,  and  so  inhibit  the  route  from releasing behind the first train. In this case  the  look  back  controls  behind  the  first automatic signal are suppressed, once the automatic signal has been  replaced  by  the passage of each train.

POINT CONTROLS

6.1    Point Setting

Point Control Tables contain the conditions which allow points to be operated and locked normal or reverse.

The "Set and Locked by Routes" entry area contains all routes that require the setting and locking of the points, whether they be in the route, in the overlap, or providing  flank  protection.

An important check in the preparation of control tables is that the routes appearing in this portion of the table include the setting of the points in their own route control tables.

6.2    Track Controls on Points

The "Tracks Locking" entry area is self explanatory for most cases. However, there are examples where more than the obvious track circuits are required.

Foul tracks must be included where  necessary,  to  protect  moves  over  the  points.  By proving the foul track clear in order to move the points away from it, this will  prevent  a fouling route being set.

Examples (Refer with respect to Figure 1 ) 

7.    ROUTE LOCKING

Once a train has passed a signal, its route can be restored but any points, opposing signals etc. ahead of the train must remain locked. This is done by the "route locking", which is indicated by the line of white lights on the signalman's panel.

On the control tables, the route locking of opposing routes and the route locking of points is shown separately (in the route and point control tables respectively). Some railways prefer to produce separate control tables for route locking.

7.1    Route Locking of Signals

Directly opposing signals with no others intervening will be locked against each other and the releasing of this locking must not occur until the train has Cleared all track circuits between the signals. Where an intervening signal occurs, the opposing signals are again  locked against each other, the locking being held until the train has cleared all track circuits in the route.

Indirectly locked signals may also require route locking. Where opposing signals do not require direct locking because they require points in opposite positions, route locking may be required if the relevant points are released as a train proceeds through the route. This locking is applied to the route setting (interlocking) controls, regardless of tracks proved dear in the signal controls, to prevent preselection of a route. No example of this situation appears on Fig 1 but if we assume that 7A track is to be divided into separate tracks over the two point ends, indirect route locking would be needed to prevent 9B route from setting after a train had passed 6 signal and cJeared the track locking 103 points.

Examples (Refer with respect to Figure 1 )

Opinion varies as to whether it is best to show route locking between opposing routes on the signal controls of the route which applies the route locking or the route which is locked by the route locking. It is more consistent with other controls to show a route as being locked by the route locking of another route and listing the track circuits required for its release. This is current TfNSW practice.

From the viewpoint of the tester, however, it is often more convenient to show the route locking which is applied by that route with its signal controls, as the contents of the whole sheet can normally be tested at the same time.


7.2    Route Locking or Shunt Routes

Route locking must always be provided where main routes lock each other or a shunt route locks a main route. However, as many shunting movements take place into occupied sections, it is impractical to apply the same conditions to many shunt routes.

The options available are:-

a)    Do not provide route locking between shunt routes where it is possible that a shunt movement into a section from one direction will be followed  by  another  in  the opposite direction to attach locomotives or vehicles.

b)    Provide the route locking as for the main routes but provide an additional release by the timed occupation of one or more track circuits after the first movement has come to a stand.

TfNSW practice is normally not to provide route locking between opposing shunt signals. The signalman is responsible for ensuring the first movement has come  to a stand  before setting the opposing route.

If a timed release was provided it would take the form shown in the table below.

Examples (Refer with respect to Figure 1 ) 

7.3    Route Locking or Points in the Route

Points are locked by setting a route through them. However, once a  train  has  passed  the signal at the entrance to the route, that signal's approach locking will be released, which potentially would release all locking on the points  ahead.  Route locking  is therefore  applied to the points to maintain the locking until the train has passed clear of the points.

Points which are situated in the overlap of a route will have  there  route  locking  released when the train is proved at a stand at the exit signal  in a similar  manner  to the timed  release of shunt route locking described above.

Route locking of points is normally shown on the points.control sheets.

Examples (Refer with respect to Figure 1 ) 


8.    OVERLAPS

As previously described main signal controls include an overlap beyond the exit signal. The length of this overlap will depend on the type of signalling,  the speed of traffic, whether  or not trainstops are fitted and whether or not the signal in rear has been conditionally cleared.

With overlaps up to 500 metres in length, the controls associated with overlaps create considerable additional complexity in the specification of controls.

The general rules for overlap controls for signals are summarised below.

a)    Facing points in the overlap are generally left as they are lying.

Exceptions to this are where there is no route forward from the exit signal in that direction, flank protection from sidings etc. would be unnecessarily removed, or the overlap in that direction is locked by a route already set but another overlap is free.

Facing points are not generally locked as they may be required to move to set the next route forward or to change to an alternative overlap (swing the overlap) due to the setting of another route.

b)    Trailing points must be set to complete the overlap according to the lie of any previous facing points (in the direction of travel).

Trailing points will be locked unless the overlap is swung clear of them.

c)    Track circuits in the selected overlap will be proved clear in the signal aspect controls.

Examples (Refer with respect to Figure 1 ) 

8.1    Overlap Point Route Locking

Trailing Points  in overlaps  are set, locked  and  detected  and  then  route  locked  in all cases. A timed release of the route locking upon the occupation  of  the  protecting  signal's  berth track allows the points to be moved once the approaching train has c.ome to a stand.

Usually facing points  may  lie either  normal  or reverse  in an  overlap.  If  the facing  points in an overlap must be locked in one position only, because the overlap to which they would lead is not permitted, then the locking on those points is the same as for trailing points.

The time of track occupation to release the  overlap  locking  will  be  dependent  on  the braking performance of trains and the length of the exit signal berth track circuit.

Examples (Refer with respect to Figure 1 ) 

8.2    Time of Operation Locking

Because facing points in an overlap are not locked, there is a danger that a train over-running into the overlap could reach points close to the signal while they are moving. Some railways attempt to reduce this risk by preventing the points from moving if the exit signal berth track is occupied. This locking would release when the train has timed to a stand.

This is often called "Time of Operation" locking. The maximum distance between the first block joint and the facing points for which time of operation locking will vary in practice according to the type of point operation. British practice uses a minimum distance  of  20 metres, extended where necessary up to approximately 50 metres.

8.3    Swinging Overlaps

Where facing points in the overlap can lie in either position creating alternative overlaps, then this is called a swinging overlap. Special controls must be provided to ensure  that once a route has been set, it will always have an overlap. The facing points  which select the alternative overlaps are generally known as the hinge points.

Where the overlap beyond the hinge points contains further points, then the locking of the hinge points is dependant on the state of the other points beyond them in the overlap. Generally if the points beyond the hinge points are lying in the correct position for the overlap or are free to move to the correct position, then they will be called and locked in the required position. If however the points beyond the hinge points are locked in the wrong position for the overlap by some other control then the hinge points will  be called  to the alternative overlap position. This indicated by an order of preference note in the control tables or by a note indicating the control being dependant on points being locked. The route locking of the points beyond the hinge points is standard for trailing points in overlaps, except that it is conditional on the lie of the hinge points.

Examples (Refer with respect to Figure 1 ) 


8.4    Overlap Maintenance

Once the entrance signal to the route bas cleared overlap maintenance locking is applied to the hinge points to ensure that the points cannot swing to an occupied alternative overlap. This locking is maintained by route locking until the train is timed to a stand on the berth track of the exit signal. During the time that the overlap is in use, the facing points can only be moved provided that the alternative overlap is available.

Examples (Refer with respect to Figure 1 ) 

8.5    Swinging by conflicting Routes

Routes which conflict with an overlap which is already set  will  set,  lock  and  detect  the hinge points to a position giving a non conflicting overlap, even though the conflicting route would not normally set the hinge points. This would also apply for two  overlaps  which conflict as well as a route conflicting  with  an overlap. The  setting  of  points  is generally done in sequence, because the hinge points must be moved to the new overlap before  the  points at the point of confliction can be released for the second route.

In other words, the order of setting must ensure that before the first overlap can be released (by swinging the hinge points), the alternative overlap bas been established.

Examples (Refer with respect to Figure 1 ) 

In the case of 111 points above, route locking from 19 signal extends only to 28HT because beyond this point overlap maintenance becomes affective.

In the case of 103 points above, the locking shown would be a first preference. The second choice would be to swing 112 normal if 103 were locked normal.


8.6    Setting by Track Circuit Conditions

Sometimes the hinge points are swung to an alternative overlap position by the occupancy of track circuits within the overlap. This is shown in the 'Set Only by Routes' entry area, with the locking being maintained by the overlap maintenance condition. However care must be taken to ensure that this does not give rise to a restrictive condition, locking out other routes. Each case needs to examined carefully on its operational merits and its affect on other routes.

EXAMPLES (Refer with respect to Figure 1)

If 121 points  are  lying  reverse  and  25Ff  is  occupied  by  a  previous  train  then  calling  121 points normal when 15(M) is set would successfully swing  the overlap  up  to 31 signal and allow 15 signal to clear. However if 25Ff remains occupied then 121 points will be locked  normal  by  the overlap  maintenance  until the train has. come to a stand  at 25 signal. During this time, the routes from 52 and 24 signal will be locked. It may have been more desirable in this case to hold the train at 15 signal and allow movements from the branch instead.

In the case of 7(M)A route swinging 111 points normal if 15CT, 25AT or (25BT w 121N) were occupied would not adversely affect other movements as the new overlap does not conflict with any other routes or overlaps.

Route Table Entries 

Point Table Entries.

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Deepu Dharmarajan - Posted 4 years ago

CH1 | THE PURPOSE OF SIGNALLING

SIGNALLING BOOK | CHAPTER 1 CONTENTS 1.Introduction 2.The Problems to be Solved 3.Basic Requirements 4.Lineside Signals 5.The Absolute Block System 6.Interlocking of Points and Signals 7.Single Lines 8.Further Developments 1. INTRODUCTION In general, the railway traveller assumes  that  his  journey  will  be safe. This  high  standard of safety which is taken for granted is the result  of  a  long  history  of  development.  As human errors and deficiencies in safety systems become evident, often as a result  of  an accident, improvements are made which are then incorporated into new generations of equipment. This is certainly true of railway signalling. It also appears to  be  a continuing  process.  We have not yet reached the situation where absolute safety can be assured. It is useful to start by looking back at some of the early history of signalling development. In the early days of railways, trains were few and speeds were low. The risk of a serious collision between two trains was minimal. Better track and more  powerful  locomotives allowed trains to run faster (requiring greater stopping distances). Railway traffic increased, requiring more and larger trains. The risks thus became greater and some form of  control over train  movements  became  necessary.  The need  for railway signalling had been identified. 2. THE PROBLEMS TO BE SOLVED 2.1 Collision with a Preceding Train When one train follows another on to the same section  of  line,  there  is a  risk  that,  if  the first train travels more slowly or stops, the second train will run into the rear of the first. Initially, trains were separated using  a system of  "time  interval"  working,  only  permitting a train to leave a station when a prescribed time had elapsed after the departure of  the previous train. Although this reduced the risk  of  collisions, a minimum safe distance between trains could not be guaranteed. However, in the absence  of  any proper communication between stations, it was the best that could be achieved at that time. 2.2 Conflicting Movements at Junctions Where railway lines cross or converge, there is the risk of two trains arriving simultaneously and both attempting to enter the same portion of track. Some  method  of  regulating  the passage of trains over junctions was therefore needed. This should ensure that one train is stopped, if necessary, to give precedence to the other. 2.3 Ensuring that the Correct Route is Set Where facing points are provided to allow a train to take alternative routes, the points must be held in the required position before the train is allowed to proceed and must not be moved until the train has completely passed over the points. Depending on the method of point operation, it may also be necessary to set trailing (i.e. converging) points to avoid damage to them. 2.4 Control of Single Lines Where traffic in both directions  must  use  the same single  line,  trains  must  not  be allowed to enter the single line from both ends at the same time. Although this could in theory be controlled by working to a strict timetable,  problems  could  still  arise if  trains were delayed or cancelled. 3. BASIC REQUIREMENTS We therefore have the basic requirements of any railway signalling system. The method of implementation has changed over the years but the purpose remains the same:- To provide a means of communicating instructions to the driver (signals) to enable him to control his train safely according to the track and traffic conditions ahead. To maintain a safe distance between following trains on the same line so that a train cannot collide with a preceding train which has stopped or is running more slowly. To provide interlocking between points and the signals allowing trains to move over them so that conflicting movements are prevented and points are held in the required position until the train has passed over them. To prevent opposing train movements on single lines. All the above requirements place restrictions on train movements, but it is vital that the signalling system will allow trains to run at the  frequency  demanded  by  the  timetable  to meet commercial requirements. This must be done without reduction of safety below an acceptable level. Signalling involves not only the provision of  equipment  but the adoption  of  a consistent  set of operating rules and communication procedures which can be understood and implemented by all staff responsible for railway operation. 4. LINESIDE SIGNALS It will probably be evident that the decisions regarding the movement of two or more trains over any portion of the railway can only be made by a person on the ground who has sufficient knowledge of the current traffic situation. His decision must be passed on to the driver of each train passing through his area of control. In the early days railways employed policemen whose duties would include the display of hand signals to approaching trains. As the policemen also had many other duties, it soon became impractical for them to be correctly positioned at all times. Fixed signals of various designs, often boards of different shapes and colours, were provided. The policeman could then set these and attend to his other duties. The simplest signals would only tell a driver whether or not he could proceed. From this evolved a standard layout of signals at most small stations; a "home" signal  on the approach side controlling entry to the station and a "starting" signal  protecting  the section of line to the next station. Between these signals, each train would be under the direct control of the policeman. These signals could give only two indications,  STOP  or  PROCEED. They therefore became collectively known as "stop" signals. As line speeds increased, "distant" signals were introduced which gave advance warning of the state of stop signals ahead. A distant signal could be associated with one or more stop signals and would be positioned to give an adequate braking distance to the first stop signal. It could give a CAUTION indication to indicate the need to stop further ahead or a CLEAR indication, assuring the driver that the stop signal(s) ahead were showing a proceed indication. With the addition of distant signals, trains were no longer restricted to a speed at which they could stop within signal sighting distance. It is important to understand the difference between stop and distant signals. A train must never pass a stop signal at danger. A distant signal at caution can be passed but the driver must control his train ready to stop, if necessary, at a stop signal ahead. The earliest signals were "semaphore" signals (i.e. moveable boards). To enable operation at night, these often had oil lamps added. With the advent of reliable electric lamps, the semaphore signal became unnecessary and a light signal could be used by day and night. Red is universally used as the colour for danger while green is the normal colour for proceed or clear. Initially, red was also used for the caution indication of distant signals but many railway administrations changed this to yellow so that there was no doubt that a red light always meant stop. If necessary, stop and distant signals can be positioned at the same point along the track. Alternatively, certain types of signal can display three or more indications to act as both stop and distant signals. 5. THE ABSOLUTE BLOCK SYSTEM Although time-interval working may seem crude, it is important to remember that nothing better was possible until some means of communication was invented. The development of the electric telegraph made the Block System possible. On many railways, time-interval working on double track lines is still the last resort if all communication between signal boxes is lost. 5.1  Block Sections In the Block Signalling system, the line is divided into sections, called "Block Sections". The Block Section commences at the starting signal (the last stop signal) of one signal box, and ends at the outermost home signal (the first stop signal) of the next box. With Absolute Block working, only one train is allowed in the Block Section at a time. The signalman may control movements within "Station Limits" without reference to adjacent signal boxes. The accompanying diagram shows a block section between two signal boxes on a double track railway. To understand the method of working, we will look at the progress of a train on the up line. Signalbox A controls entry to the block section but it is only signalman B who  can see a train leaving the section, whether it is complete (usually checked by observation of the tail lamp) and who thus knows whether or not the section is clear. Signalbox B must therefore control the working of the UP line block section. Similarly, signalbox A controls the DOWN line block section. 5.2 Block Bell The signalmen at each end of a block section must be able to communicate with each other. Although a telephone circuit is a practical means of doing this, a bell is normally used to transmit coded messages. It consists of a push switch ("tapper") at one box, operating a single-stroke bell at the adjacent box (normally over the same pair of wires). The use of a bell enforces the use of a standard set of codes for the various messages required to signal a train through the section and imposes a much greater discipline than a telephone, although a telephone may be provided as well, often using  the same circuit as the block bell. 5.3 Block Indicator This provides the signalman at the entrance to the  section with a continuous visual indication of the state of the section, to reinforce the bell codes. It is operated by the signalman at the exit of the block section. Early block instruments were "two position" displaying only two indications;  line clear and line blocked. Later instruments display at least 3 indications. The most usual are:- Line clear Giving permission to the rear signalman to admit a train to the section. Normal or Line Blocked Refusing permission. The signalman at the entrance to the section must maintain his starting signal at danger. Train on Line There is a Train in the block section. 5.4 Method of Working When signalbox A has an UP train approaching to send to box B,  the signalman at A will offer it forward to box B, using the appropriate bell code (so that signalman B knows what type of train it is). If the signalman B is unable to accept the train for any reason, he will ignore A's bell, and leave the UP line block indicator at "Normal". If he is able to accept the train, signalman B will repeat the bell code back to box A, and change the indication to "Line Clear". When signalman A sees  his  block  repeater go to "Line Clear", then he can clear his starting signals to admit the train to the section. When the train actually enters the section, signalman A sends the "Train Entering Section" bell code to box B. Signalman B will acknowledge this by repeating the bell code back to A, and turning the block indicator to "Train on Line". When the train leaves the block section at B, the signalman  there checks  that it is complete by watching for its tail lamp. He then turns his block indicator to "Normal" again.  He also sends the "Train out of Section" bell code to A, which A acknowledges by repeating it back. The system is now back to normal, ready for the next train. On multiple track railways, a pair of block instruments as above is required for each line. 5.5 Extra Safeguards The basic three-position block system, as described, relies on the correct sequence of operations for safety. A signalman could forget that he has a train in section and turn the indicator to "line clear", allowing a second train in. A detailed record (the train register) is kept of the actual times of train arrival and departure, and the times at which the bell signals are exchanged. In most places, additional safeguards have been added to the basic system. An  electric lock on the starting signal will prevent it being operated unless the block indicator is at line clear. Track circuit occupation may be used to set the block  instruments to ''Train on Line" if the signalman forgets to do so. Electric locking may also be used to ensure that signals are operated for one train movement only and replaced to danger before another movement is permitted to approach. Although it is unusual for absolute block working to be installed on any new signalling installation today, there are many railways on which it is in widespread use. The  block system, by ensuring that only one train may occupy a section of line at any time, maintains a safe distance between following trains. 6. INTERLOCKING OF POINTS AND SIGNALS On all early railways, points were moved by hand levers alongside the points. They could therefore be moved independently of the signals controlling the movement of trains. A great improvement in safety (as well as efficiency) was possible by connecting the point switches via rodding to a single central control point (the signal box). Similarly the signals could also be operated by wire from levers in the signal box. With the control of points and signals all in one place the levers  could be directly interlocked with each other. This had the following benefits:- Signals controlling conflicting routes could not be operated at the same time. A signal could only be operated if all  the points were in the correct position. The points could not be moved while a signal reading over them was cleared. In early signalling installations, all point and signal operation, together with any interlocking, was mechanical. Although it was a great technological advancement to be able to control a station from one place, the effort required to operate the levers restricted control of points to within about 300 metres from the signal box and signals up to about 1500 metres. At large stations, more than one signal box would often be necessary. The possibility still existed for a signalman to set the points, clear the signal, the train to proceed and then for the signalman to replace the signal to normal. This could free the locking on the points before the train had completely passed over them. Signalmen's instructions usually required the complete train to pass over the points before the signal was replaced to danger. 7. SINGLE LINES On most single line railways trains are infrequent. It is not normally necessary for two trains to follow each other closely in the same direction. Single lines were therefore treated in the same way as a normal block section with the important extra condition that trains could not be signalled in both directions at the same time. To enforce this condition and also to reassure the driver that he could safely enter the single line, some form of physical token was used as authority to travel over the single line. On the simplest of systems only one token existed. This caused problems whenever the pattern of service differed from alternate trains in each direction. If the timetable required two trains to travel over the single line in the same direction, the driver of the first train would be shown the token (or train staff  as  it is commonly  known) to assure the driver that no other train was on the single line. His authority to enter the single line would however be a written ticket . The following train would convey the train staff. Although workable, this system would cause problems if trains did not work strictly to the timetable. A further improvement was to provide several tokens, but to hold them locked in instruments at either end of the single line. The instruments would be electrically interlocked with each other to prevent more than one token being withdrawn at a time. The one token could however be withdrawn from either instrument. If the single line block equipment fails, many railways employ a member of the operating personnel as a human token. The "pilotman", as he is usually known, will either travel with the train or instruct the driver to pass through the section. No other person may allow a train on to the single line. Operationally, this is the equivalent of the train staff and ticket system described earlier. 8. FURTHER DEVELOPMENTS The main functions of the signalling system had now been defined, although they were to be continuously improved as the available technology developed. All  signalling  systems would be required to maintain a safe distance between trains, interlock points and signals and thus prevent conflicting movements, and provide the necessary information so that the speed of all trains can be safely controlled. In recent years, the signal engineer has been asked to provide further facilities within the general scope of the signalling system. These include, train information to the operating staff, train information for passengers, detection of defective vehicles, identification of vehicles and the increasing automation of tasks previously carried out by humans. The technology exists to completely operate a railway without human intervention although the level of automation desirable for a particular railway is for that railway administration to decide. Factors such as cost, maintainability, reliability, staffing policy, passenger security and sometimes political considerations must be taken into account. In many cases the final decision on the type of signalling to be provided is outside the direct control of the signal engineer. However, he should always endeavour to provide the best possible information and propose cost-effective solutions to particular problems so that the best decisions can be made.

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Deepu Dharmarajan - Posted 4 years ago

CH2 | BASIC SIGNALLING PRINCIPLES | PART 1

SIGNALLING BOOK | CHAPTER 2 | PART 1 CONTENTS 1. Introduction - In Part 1 2. Signal Aspects - In Part 1 3. Signalling Principles - In Part 2 4. Drawing Standards - In Part 2 5. Interlocking Principles - In Part 2 6. Train Detection & Track Circuit Block - In Part 2 7. Colour Light Signals - In Part 2 8. Control Panels & Other Methods of Operation - In Part 2 9. Colour Light Signalling Controls - In Part 2   1. INTRODUCTION Whatever type of signalling system is provided on a railway, its basic functions will remain the same. Safety must be ensured by preventing trains colliding with each other and locking points over which the train is to pass. The means of achieving these functions may vary from one railway administration to another but a set of rules must be laid down to define:- The positioning of signals The types of signals The aspects to be displayed by the signals and the instructions to be conveyed  by those aspects The controls to be applied to the signals The method of controlling points The method of interlocking points with signals The standardisation of human interfaces Many countries have sytems of signalling based on British railway signalling practice. The basic British system is very simple having only a small number of different signal aspects displayed to the driver. The driver is responsible for knowing the route over which he is to pass. The signal engineer must, in turn, provide sufficient information for the driver to safely control the speed of his train and, where necessary, to inform him which route he is to take. Other signalling systems have developed along a different path. The driver is given specific instructions to travel up to or reduce to an indicated speed. Route indications are optional. This will generally require a more complex set of signal aspects. This section will deal mainly with the principles and practices of the State Rail Authority of New South Wales, with reference to other systems where appropriate. 2. SIGNAL ASPECTS There are three principal types of signal, each serving a different purpose:- Main or Running signals control the normal movement of passenger and freight trains on running lines. The great majority of movements will be controlled by main signals. Subsidiary signals, mounted on the same post or structure as running signals, control movements other than for normal running, such as the shunting or coupling of trains. Independent shunting signals, generally similar to the subsidiary signals above, are provided for shunting movements at positions where there is no need for a running signal. We will examine the aspects displayed by each type of signal and the instructions and/or information conveyed by them. 2.1 Main or Running Signals As all early signals were semaphore signals, displaying a light for night time use, the aspects of colour light signals are usually based on the indications of the semaphore signals which they replaced. As most new signalling installations are likely to employ colour light signals, this section will concentrate on colour light signalling only. SRA (Currentlly TfNSW) employs two methods of signalling on main lines; single light and double light. As the name suggests, double light signals will always display at least two lights to the driver. Double light signalling is generally used in the Sydney metropolitan area. Single light signals normally use only one light to convey instructions to the driver, although a second marker light may be illuminated to aid the driver in locating the signal. Single light signalling is mostly used on lines outside the Sydney metropolitan area. Although there are similarities between the two systems, we will deal with each separately. We will then make a comparison with the corresponding aspects displayed by the British system to enable readers to read signalling plans drawn in British style. 2.1.1 Double Light Signalling This is intended for use in areas where signals are closely spaced. Each stop signal is therefore required to carry a distant signal for the signal ahead. To give the driver a consistent indications, each signal carries two separate signal heads. The upper signal head can be considered as the stop signal. It will always be capable of displaying, at least, stop and proceed aspects. The lower signal head can be considered as the distant for the next signal ahead. An additional green light may be provided below the distant. This is used for a "low speed" indication . FIGURE 1 shows the normal running aspects for double light signalling. Four aspects are used for normal running :- STOP is denoted by two red lights, one above the other. Note that the lower signal head will always display a red if  the upper signal is at red, even if the signal ahead is showing a proceed aspect. This is important to avoid misleading or confusing the driver. CAUTION is denoted by green over red. In other words, this signal is at "proceed" (top signal head) but the next signal is at "stop" (bottom signal head acts as distant). The caution indication tells the driver to be prepared to stop at the next signal. MEDIUM is a preliminary warning of the need to stop. It is denoted by green over yellow. Signals in urban areas may be closely spaced.  The one  signal  section between the caution and the stop may provide insufficient braking distance for a train travelling at full line speed. The medium indication tells the driver that the next  signal is at caution. This implies that he may have to  stop  at  the  second  signal ahead. CLEAR allows the train to proceed at maximum speed. A clear indication is two green lights. This will tell the driver that there is no need to reduce speed (other than for fixed speed restrictions) before the next signal. All the above indications require the driver to know where the next signal is, to safely control the speed of his train and be able to stop where required. An additional indication is provided on some signal, A  LOW  SPEED  indication, consisting of a small green light below a normal stop aspect tells the driver to proceed at no more than 27km/h towards the next signal. This is generally more  restrictive than the  caution. The low speed aspect is used when the track is only clear for a very short distance beyond the next signal. Fig 1: DOUBLE LIGHT SIGNALLING - ASPECTS FOR NORMAL RUNNING *Where a low speed indication is provided. Fig 2: DOUBLE LIGHT SIGNALLING - TURNOUT ASPECTS NOTE: A full clear indication is not given for turnouts. Note the difference in the indications given by Multi-light signals at a turnout. The yellow over red indicates "Proceed" at Medium speed through Turnout, next signal at "Stop". The Yellow over Yellow indicates "Proceed at Medium speed through Turnout" the lower Yellow is cautioning the driver to continue at Medium Speed towards the next signal which is indicating either "CAUTION" or "CLEAR" *Where a low speed indication is provided. **In the Sydney and Strathfield resignaled areas this indication represents a 'low speed' with  the train stop at stop. In this case the signal in the rear will show a caution indication. Figure 2 shows the indications for double light turnout movements. If there is more than one route from a main signal, the driver  must be told whether he is to take the main line or through route or whether he is to take a lower speed diverging turnout. This information is necessary to prevent the driver running through a turnout at too high a speed. The upper signal head is used to display a distinct proceed aspect for a turnout. Instead of the green normally displayed, a yellow light will tell the driver that he is to take the turnout. The proceed aspects for turnouts are:- CAUTION TURNOUT (yellow over red, also described in some operating documents as medium caution) tells the driver to expect the next signal to be at red. MEDIUM TURNOUT (yellow over yellow) tells the driver that the  next  signal ahead is displaying a proceed aspect. This is the least restrictive aspect for a turnout. There is no equivalent of a clear aspect for trains signalled over a turnout. To give a clearer indication to the driver where several routes are possible from one signal, the main signal aspect may be used instead, in  conjunction with a theatre route indicator. The route indicator contains a matrix of small  lunar  white lights which  can  be illuminated to display a Jetter or number. Each character will be associated with a distinct route. The route indicator is not illuminated when the signal is at stop.  When  the  signal  is required. to clear, the route indicator will illuminate for the appropriate route. LOW SPEED aspects may be used. If  a  low  speed  aspect is provided for a turnout route, this is no different in appearance to a  low speed for the main line route. This is not a problem as this aspect conveys a specific speed instruction. As the speed is  likely to be lower than that of most turnouts, route information is not essential 2.1.2 Single Light Signalling On lines where single light signalling is installed, the spacing of signals may vary widely. Therefore some signals may be combined stop and distant signals (as for double  light) but there may also be signals which are stop signals only or distant signals only. The instruction to the driver is therefore generally conveyed by a single light. A second marker light is provided below the main light to aid the driver in locating the signal. The meaning of the signal aspects is equivalent to the double light aspects but the appearance to the driver is different. FIGURE 3 shows the normal running aspects  for single  light signalling. The appearance of each aspect is as follows:- STOP consists of a red light. The marker light also displays a red, except on some older signals where it is lunar white. CAUTION consists of a single steady yellow light. The marker light is extinguished, except on some older signals where it is lunar white. If the main light should fail the marker light  will  display a red on stop signals or yellow on distant signals. MEDIUM, where this aspect is necessary, will be a flashing or pulsating yellow light. The marker light will operate as for the caution aspect. CLEAR is a green light. The marker light will operate as for the caution aspect. LOW SPEED aspects may be used in single light signalling where required. An additional small green light is provided below the marker light. The complete low speed aspect will be a main red light over a red marker light with the additional green light illuminated . Fig 3: SINGLE LIGHT SIGNALLING - ASPECTS FOR NORMAL RUNNING   Fig 4: SINGLE LIGHT SIGNALLING - TURNOUT ASPECTS The indication displayed by a Home signal for a turnout movement through facing points into a Loop Refuge siding or important siding consists of a band of three yellow lights in a subsidiary light unit (inclined towards the direction  of the movement). The Red light is  displayed in the Main line signal , as shown. The marker light for the main line signal, contained in the subsiduary light unit will be extinguished when the main line or turnout signal indication is displayed. 4.1 ROUTE INDICATIONS MAIN LINE At locations where more than one turnout is provided one signal indication is some times given and in such cases a route indicator working in conjunction with th e signal is provided, this enables drivers to ascertain the route for which th e signal has been cleared. The route indicator will not show any indication when the signal is at stop, but when the points have been set for the turnout movement a yellow light will appear in the signal in conjunction with the route indication showing a letter to denote the line to which the train will travel, e.g. Figure 4 shows the indications for single light turnout movements. For junction signals, two distinct methods are used according to the situation. For a simple turnout into a loop or siding, a separate turnout signal is provided below the main aspect, incorporating the marker light. For a CAUTION TURNOUT aspect, the main aspect remains at red, the marker light is extinguished and the three yellow lights of the turnout signal are illuminated . The row of lights is inclined in the direction of the turnout. For a MEDIUM TURNOUT aspect, the turnout signal will flash. Otherwise the appearance is the same as above. As for double light signalling, a theatre route indicator may be used in conjunction with the main signal aspect, where several routes are possible from one signal. Again the route indicator is not illuminated when the signal is at stop. When  the signal  is required to clear, the route indicator will illuminate for the appropriate route. The normal construction of signals is to provide a separate lamp unit for each light to be displayed . Some signals, however, are of the "searchlight" type. In this type of signal, the lamp is continuously illuminated and coloured lenses are moved in front of the  lamp according to the aspect to be displayed. The lenses are moved by a relay mechanism inside the signal head. The lights visible to the driver are the same for either type of signal. 2.2 Subsidiary Signals Associated with Main Signals As well as normal running movements, signals may be required for some of  the following movements:- Entering an occupied section Shunting into a siding Running on to a line used for traffic in the opposite direction. Attaching or detaching vehicles or locomotives. The main types are described below. As for the running signals, only current practice is covered in detail. Although the SRA (TfNSW) practice will allow subsidiary signals to clear immediately the route is set, many railway administrations employ approach control to delay clearance of the subsidiary signal until the train has come at or almost to  a  stand  at  the  signal. This  is usually achieved by timed track circuit occupation. The driver will receive a caution at the previous signal and will be preparing to stop. Subsidiary signals are short range signals which are only visible within a short distance of the signal. 2.2.1 Subsidiary Shunt and Calling-on Signals These authorise a driver to pass a main signal at stop for shunting purposes or to enter an occupied section. The driver must be prepared  to  stop short of  any  train or other obstruction on the line ahead. He must therefore control the speed of his train so that he can stop within the distance he can see. The appearance of these signals is a small yellow light below the main aspect. On  some older double light signals the letters "CO" in a round lens illuminated in white may be used. 2.2.2 Shunt Ahead Signal This signal is generally found on single and double lines worked under absolute block conditions. It permits the movement of a train past the starting signal for shunting purposes. It does not require a block release from the signal box ahead and the movement  will  eventually come back behind the starting signal when shunting is complete. As this method of working is generally only found outside the suburban area, a shunt ahead signal will normally be provided on single light signals only. It consists of a small flashing yellow signal below the main running signal and marker light. 2.2.3 Close-up Signal This is similar in appearance and application to the low speed signal. 2.2.4 Dead-end Signal This is for entering short dead end sidings directly from a running line. The only difference between this and a subsidiary shunting or calling-on signal is that the dead end signal is offset from the post on the same side as the siding leads off the main line. Subsidiary signals display no aspect when not in  use.  The  associated  main  signal  will  remain at stop when the subsidiary signal is in use. Route indicators may be used in conjunction with subsidiary aspects to give an  indication to the driver where multiple routes are available. In the case of a movement on to another running line in the wrong direction (i.e. opposite to normal direction of traffic) a route indication is always provided. 2.4 Shunting Signals Signals may also be required for shunting movements in positions where no main signal is necessary. The most common locations are:- Entrance to and exit from sidings. At crossovers to allow a wrong road movement to regain the right line. In yards and depots where main signals are not required. As they have no associated main signal, they must display a stop as well as a proceed aspect. 2.5 Dwarf and Position-light Signals Two main types are in use, the dwarf signal, with all lights arranged vertically and the position light signal. The diagrams show the various signal profiles.  Both types display two red lights for stop. The proceed aspect is normally only a yellow indicating caution. Shunting signals do not always prove track circuits clear and the driver must be ready to stop if there is another train occupying the section ahead. The proceed aspect may be accompanied by a route indication. Shunting signals are normally mounted at ground level, although they may be elevated if required for sighting. 2.3.2 Stop Boards If a wrong road movement is authorised from a shunting or subsidiary signal there must be another signal ahead to limit the wrong road movement. If no such signal was provided, the movement could continue past the protecting signal for the normal direction of traffic and cause a collision. The usual signal is an illuminated notice board carrying the words "SHUNTING  LIMIT". It can be considered as a shunting signal permanently at danger. 2.3.3 Facing Shunt Signals A shunting signal is sometimes needed in a position where it is passed in the normal direction by running movements. To avoid confusing  the driver by displaying a yellow light for a movement which may well be running under the authority of clear signals, these facing shunt signals are provided with an additional green light (clear aspect) for use only in this situation. 2.3.4 Point Indicators Although not signals in the same way as those just described, point indicators are important in sidings to avoid derailments and damage to equipment. They provide a visible indication of the position of hand operated points. Operation may be either mechanical or electrical. Located alongside the point switches, they display an illuminated arrow in the direction of the line for which the points are set. 2.5 British Signalling Aspects For the benefit of those who may at some time have to read signalling plans drawn to British standards (e.g. for the IRSE examination), this is a brief summary of the aspects in use, their meanings and how they are drawn. 2.5.1 Main Signals As with SRA (Currently TfNSW) practice, three colours are used, red,  yellow  and  green.  On  the  plan,  a  red light is denoted by a circle with a horizontal line across it. A yellow  light  has the line at 45° and a green light has a vertical line. The  "normaJ"  aspect  of  the signaJ  (i.e. with  no  routes set and all track circuits clear) is shown by a double line in the appropriate light(s). There are four available aspects; STOP is a red light, CAUTION is a single yellow light, PRELIMINARY CAUTION is two yellow lights and  CLEAR  is  green.The stop, caution and clear signals have the same meanings as the corresponding SRA  aspects. The preliminary caution is similar to the MEDIUM indication  of  SRA  signaJs. The  double yellow aspect is only used in situations where signals  must  be  positioned  closer  together than braking distance. Many lines use red, single yellow and green only. Marker lights are not used. There is no equivalent of the LOW  SPEED  signal. An equivalent control (to allow trains to close up provided they are running  at very low speed)  is provided  by delayed clearance of the yellow aspect. The train must be almost stationary at the signal before the aspect will change from red to yellow. This is achieved by applying approach control with timed track circuit occupation. 2.5.2 Junction Signalling Where a signal has more than one route, a distinct route indication must be given for each  route, except that the highest speed or straight route  need  not have a route  indication.  This may take one of two forms, a junction indicator (a row of  five white lights)  normally  above the main signal and pointing in the direction of the divergence or a multi lamp or fibre optic route indicator displaying one or two characters. There are six available junction indicator positions. Positions 1, 2 and  3 (at 45°, 90" and 135° respectively) indicate diverging routes to the left. Positions 4, 5 and 6 provide equivalent indications to the right. Multi-lamp or fibre optic route indicators are restricted to routes with a speed of 40 mph (64 km/h) or less. Where necessary, clearance of the junction signal is delayed by occupation of the approach track circuit (timed if necessary) to enforce a speed  reduction.  This  is  because  the  driver may receive no warning at previous signals of the route set from the junction signal 2.5.3 Subsidiary Signals The standard subsidiary signal is a position light with two white lights at 45°. The proceed aspect is both lights illuminated. There is no stop aspect -  the  associated  main  signal remains at red. Route indicators are provided  where  necessary but are not obligatory -  if they are provided, route  indications  must  be displayed  for  all routes. The subsidiary signal is used for all shunting and calling on moves. This is a short range signal. An approaching train must be  brought  to a stand  before clearance of the subsidiary aspect. 2.5.4 Shunting Signals The position light shunting signal has two white lights and one red  light. The proceed  aspect is identical to the subsidiary signal. The stop aspect is one red and one white  light, horizontally placed. The white light at the lower right {the "pivot" light) therefore remains continuously lit. A shunting signal with two red lights only is used as a "limit of shunt" indicator. 2.6    Summary Whatever the system of signalling, the signal engineer  must  have  a  detailed  knowledge  of the aspects displayed to the driver and the instructions conveyed. He must then design  the layout of the signalling and the associated controls so that the driver can safely obey all signal aspects. This  must apply for all types of  train likely to use a line. When  required  to reduce  speed or stop, trains must have adequate braking distance under all conditions. The driver of a train must never be given an instruction by a signal that he is unable to comply with.   TO BE CONTINUED - SIGNALLING BOOK | CHAPTER 2 | PART 2...........

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Deepu Dharmarajan - Posted 4 years ago

CH3 | SIGNALLING A LAYOUT | PART 1

SIGNALLING BOOK | CHAPTER 3 | PART 1 CONTENTS 1. Introduction - In Part 1 2. Headway - In Part 1 3. Positioning of Running Signals - In Part 2 4. Types of Signal - In Part 2 5. Points and Crossings - In Part 3 6. Track Circuits - In Part 3 7. Identification of Signals, Points & Track Circuits - In Part 3 8. Examples - In Part 3 1. INTRODUCTION One of the first steps in any signalling project is to determine the method of train working. Having decided this, it is then necessary to decide the position and spacing of signals. This section will assume throughout that colour light signalling to track circuit block principles will be provided on all main lines. Although other methods of working may well be more appropriate, particularly for lightly used single lines, these will be covered later in the course. It is useful at an early stage to determine whether 2, 3 or 4 aspect signalling will be required. This will be governed by the required line capacity, which in turn will be determined by the timetable to be operated. Having this information and an approximate signal spacing, we can then proceed to position the signals on a scale plan of the track layout. Their position relative to stations, junctions etc. will be decided largely by operating requirements. The most economical arrangement that meets all operating requirements is the one that should be adopted. In order to produce a safe and economical signalling scheme, the designer must use his knowledge of signalling principles and be provided with all necessary details of the train service pattern required, the track layout, gradient profiles, line speeds and train characteristics. If this information is not immediately available, it must be requested from the appropriate authority. Sometimes operating requirements conflict with each other and with safety standards — the engineer must then use his experience to reach a satisfactory compromise whilst maintaining the safety standard. 2. HEADWAY The headway of a line is the closest spacing between two following trains, so that the second train can safely maintain the same speeds as the first. This usually means that the second train is sufficiently far behind the first that its driver does not see an unduly restrictive signal aspect. Headways can be expressed in terms of distance but more usefully as a time (e.g. 2 1/2 minutes between following non-stop trains). It can also be converted to a line capacity (trains per hour). Care must be taken when using a "trains per hour" figure if the trains are not evenly spaced in the timetable. The signalling must be able to handle the minimum headway, not the average. Headway will depend on a number of factors:- D = Service Braking Distance d = Distance between STOP signals S = Sighting Distance (usually 200 yds/metres or distance travelled in 10 seconds) O = Overlap Length L = Train Length (less than 100 yds/metres for a short suburban train but possibly over 1km for a heavy freight train) V = Line Speed (or actual train speed if lower) a = Braking rate Where any of these factors are not given to you, you should always state your assumptions. In practical situations, it is vital to obtain accurate information regarding the braking performance of trains. It is also vital to standardise your units of distance and time. If you work in imperial, yards and seconds are most useful; in metric, metres and seconds would be most appropriate. Whichever you decide, you must use the same set of units consistently throughout to avoid confusion and error. 2.1. Service Braking Distance This is the distance in which a train can stop without causing undue passenger discomfort. It will depend on the line speed, gradient, and type of train. It is usually significantly greater than the emergency braking distance. Theoretically, the Service Braking Distance can be calculated using the line speed and braking rate         This is derived from the 3rd Law of Motion. This calculation will depend upon the braking characteristics of the type(s) of train using the line and must take into account the worst case combination of train speed and braking rate. If this calculation is to be performed frequently, it is useful to show the service braking distances for different combinations of speed and gradient in tabular or graphical form. Gradient should always be taken into account. A falling gradient will increase braking distance, a rising gradient will reduce it. As gradients are rarely uniform between signals, we need to calculate an average gradient using the formula: where G is the average gradient   D is the total distance    g and d are the individual gradients & distances. For a gradient of 1 in 100, G = 100. If the gradient is expressed as a percentage, G is the reciprocal of the percentage gradient. Falling gradients taken as negative, rising gradients as positive. 2.2.  2 Aspect Signalling 2 aspect signalling will generally be adequate on lines where traffic density is low. The required length of block section is much greater than braking distance. Only two types of signal are used, a stop signal showing stop and clear only and a distant signal showing caution or clear. Each stop signal will have its associated distant signal. As 2 aspect signalling will mainly be found outside the suburban area, the example shows single light signals. The distance (d) between stop signals is variable according to the geography of the line, positions of stations, loops etc. The headway distance can be calculated as: H = D + d + S + O + L giving a headway time:         Note that the headway time for the line is that of the longest section and cannot be averaged. To obtain the greatest signal spacing to achieve a specified headway, we transpose the equation to give: d = (V x T) - ( D + S + O + L)   2.3.  3 Aspect Signalling With 2 aspect signalling, as the required headway reduces, each stop signal will become closer to the distant signal ahead. it is therefore more economic to put both signals on the same post. This then becomes 3 aspect signalling. Each signal can display either stop, caution or clear. The distance (d) between signals must never be less than braking distance (D), but to ensure that the driver does not forget that he has passed a distant at caution, (d) should not be excessively greater than the service braking distance. The current SRA recommendation is for signal spacing to be no greater than three times braking distance. BR has adopted a maximum of 50% (i.e. 1.5D) although this is often exceeded at low speeds. The headway distance is given by:- H = 2d + S + O + L So the best possible headway, when the signals are as close as possible (exactly braking distance), is: H = 2D + S + O + L The headway with signals spaced 50% over service braking distance is: H = 3D + S + O + L The headway with signals spaced at three times braking distance is: H = 6D + S + O + L   2.4.  4 Aspect Signalling Where signals are closer together than braking distance, a preliminary caution or medium aspect is needed to give trains sufficient warning of a signal at danger. This medium aspect must not be less than braking distance (D) from the stop aspect, so the distance (d) between successive signals must on average be no less than 0.5D. The headway distance is given by:- H = 3d + S + O + L       where d > 0.5 D  So the best possible headway with 4 aspect signalling is given by:- H = 1.5 D + S + O + L In practice, the geographical constraints of the track layout will probably prevent regular spacing of signals at 0.5D. If the total length of two consecutive signal sections is less than braking distance, an additional medium aspect will be required at the previous signal. In other words, the first warning of a signal at stop must be greater than braking distance away. If more than two warnings are required, the medium aspect is repeated, not the caution. Signals should however be positioned so that this situation is as far as possible avoided. 2.5. Application of Low Speed Signals and Conditional Caution Aspects In normal use, the addition of a low speed signal provides the driver with a fifth aspect. It is important to realise that this does not have any effect on the headway of through or non-stopping trains running at their normal speed. In this situation, the engineer will arrange the signals so that each driver should, under normal conditions, see only clear aspects. The preceding headway calculations apply regardless of whether low speed signals are provided or not. A low speed signal tells the driver that he has little or no margin for error beyond the next signal and should control the speed of his train accordingly. The benefit of low speed signals is in allowing a second train to close up behind a stationary or slow moving train by reducing the length of the overlap, provided the speed of the second train has been sufficiently reduced. The same effect can be achieved by delaying the clearance of the caution aspect. This is now preferred, provided an overlap of the order of 100 metres can be achieved. The clearance of the signal should be delayed to give a passing speed of approximately 35km/h. Low speed signals should only be used where the reduced overlap is very short (less than 50 metres) and/or there are fouling moves within 100 metres of the stop signal. 2.5.1. Station Stops With an overlap of 500 metres, a train stopped at a station will have at least 500 metres of clear track behind it. A following train will stop at the first signal outside this distance. By the addition of a low speed signal or a conditionally cleared caution, the overlap distance can be reduced and the second train can approach closer to the station. When the first train leaves the station, the second train can enter the platform earlier, thus giving a better headway for stopping trains. A conditionally cleared caution aspect will normally be used unless the overlap is less than 50 metres. 2.5.2. Approaching Junctions Trains awaiting the clearance of another movement across a junction can approach closer to the junction while keeping the overlap clear of other routes across the junction. A low speed aspect will normally be used in this situation. 2.5.3. Recovery from Delays A line which is operating at or near its maximum capacity will be susceptible to disruption from even minor train delays (e.g. extended station stops at busy times). Low speed signals and or conditionally cleared caution aspects can allow trains to keep moving, even if only slowly, to improve recovery from the delay. The total length of a queue of trains will be less and the area over which the delay has an impact will be reduced. 2.6. Summary For 2 aspect signalling, the headway distance is:- H = D + d + [S + O + L]   For 3 aspect signalling, the headway distance is:- H = 2D + [S + O +  L]   (minimum) where signals are spaced at braking distance H = 2d + [S + O+ L] (general case) for an actual signal spacing of d   For 4 aspect signalling, the headway distance is:- H = 1.5 D + [S + O + L] (minimum) where signals are spaced at braking distance H = 3d + [S + O +  L]  (general case) for an actual signal spacing of d   Note the factor [S + O + L] is common to all equations.   Headway time is then calculated as:         2.7. Determining Signal Type and Spacing Because cost is generally proportional to the number of signals, the arrangement of signalling which needs the smallest number of signals is usually the most economic. It must, however, meet the headway requirements of the operators. For non-stop headways it is likely that the same type of signalling should be provided throughout. Otherwise there will be large variations in the headway. Remember that the headway of the line is limited by the signal section which individually has the greatest headway. This section will briefly describe a technique for determining the optimum signalling for a line. There may need to be localised variations (e.g. a 2 aspect signalled line may need 3 aspect signals in the vicinity of a station or a 3-aspect line may need to change to 4 aspect through a complex junction area). These variations will depend on the requirements for positioning individual signals and can be dealt with after the general rules have been determined. Firstly, determine braking distance, train length and overlap length required. Each must be the worst case. Knowing the required minimum headway, use the H = 2D + S + O + L equation to determine the best possible headway for 3 aspect. Compare the results with the required headway to check whether "best case" 3 aspect signalling is adequate. There should be a margin of 25–30% between the theoretical headway and that required by the timetable to allow for some flexibility to cope with delays. 2.7.1. If the Headway is Worse than Required 3 aspect will not be adequate and 4 aspect must be used. Recalculate for 4 aspect to confirm that this does meet the headway requirement. T = (1.5D + S + O + L) / V If the non-stop headway requires 4 aspect signalling, it is likely that station stops will cause further problems. Signal spacing near stations should be kept to a minimum and low speed signals or conditionally cleared cautions with reduced overlaps may also be required. 2.7.2. If the Headway is Much Better Much better generally means a headway time of 30% or less than that required by the timetable. If this is the case 2 aspect will generally be adequate. Calculate the greatest signal spacing that will achieve the headway with 2 aspect signalling. d=(V x T) - (D + S + O + L) Remember that in this distance d there will be two signals, a stop signal and a distant signal. Then compare this with the maximum permissible signal spacing for 3 aspect. In the absence of any firm rules, a judgement must be made on the amount of excess braking which is acceptable. SRA recommends that signal spacing is no more than three times braking distance while BR signalling principles specify no more than 1.5 times braking distance. If the two calculations produce a similar total number of signals (i.e. d for 2 aspect is approximately twice the value of d for 3 aspect) a 3 aspect system will be the better choice. The cost of the signals will be similar and the operator may as well benefit from the improved headway provided by 3 aspect. 2.7.3. If the Headway is Slightly Better It is probable that 3 aspect is the correct choice. Check that there is sufficient margin between the required and theoretical headway. 2.7.4. Signal Spacing Having evaluated that the chosen arrangement of signalling will provide the required headway, the relevant equation should be transposed to calculate the greatest possible signal spacing that can be allowed with the specified headway: eg. for 3 aspect signalling: V x T = H           = 2d + S + O + L therefore  2d     = (V x T) - (S + O + L) from which the post to post spacing (d) can be calculated Remember, there may be a constraint on the maximum signal spacing. The value of d should not exceed this. Geographical constraints may also require signals to be closer together than braking distance, in which case the 4th (medium) aspect is used where required. It does not need to be used throughout unless for headway [puposes]. 2.8. Example Information given:- Max. Line Speed...... 90 km/h Gradients ..........  Level Train Length.............. 250 metres Headway Required..... 2 1/2 mins. (non-stop) Before we start, we need the Service Braking Distance, either by calculation or from tables/curves (where available). We will assume that D = 625 metres. Note : S assumed to be 200 metres. O assumed to be 500 metres (although overlaps may need to be more accurately calculated if trainstops used) V = 90 km/h = 25m/s First, check 3 aspect signalling:- H = (2D + S + O + L) = (1250 + 200 + 500 + 250)  = 2200 metres so T = H/V = 88 seconds. This is much less than the 150 seconds (21/2 mins) specified. We will therefore consider the alternative of 2 aspect signalling. We cannot calculate a theoretical headway for 2 aspect signalling as the signal spacing is not fixed. Instead, we calculate the greatest 2 aspect signal spacing to give us the 150 second headway specified : d = (V x T) - (D + S + O + L) = (25 x 150) - (625 + 200 + 500 + 250) = 3750 - 1575 metres = 2175 metres Hence 2 aspect signalling, with the stop signals no more than 2175 metres apart, would give the 2 1/2 min. headway required. However, each stop signal also requires a distant signal. Two signals are therefore required every 2175 metres. 3 aspect signalling with signals every 1088 metres would require no more signals but would give a better headway of: H = 2d + (S + O = L) = 2175 + (200 + 500 + 250) = 3125 metres So T = H/V = 3125/25 = 125 seconds In fact, the signal spacing could be extended further within the headway requirement of 150 seconds. This would give a better headway with fewer signals than 2 aspect. This demonstrates that 2 aspect is generally worth considering only for very long headways. We could now calculate the maximum possible 3 aspect signal spacing allowed by the headway : V x T = H = 2d + S + O + L therefore 2d      = (V  x  T) - (S + O + L) = (25 x 150) - (200 + 500 + 250) = 3750 - 950 = 2800 metres d        = 1400m As this is over twice braking distance, it should be confirmed that this signal spacing is operationally acceptable TO BE CONTINUED - SIGNALLING BOOK | CHAPTER 3 | PART 2...........

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