CH3 | SIGNALLING A LAYOUT | PART 2
Signalling
CONTINUED FROM - SIGNALLING BOOK | CHAPTER 3 | PART 1
SIGNALLING BOOK | CHAPTER 3 | PART 2
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
3. POSITIONING OF RUNNING SIGNALS
If we are starting with a blank track layout, we need a logical method of setting out the signals in order to produce a signalling plan. As running signals must fulfil the needs of both headway and braking distance, it is usual to position running signals first. Shunting and subsidiary signals are dealt with after the running signals have been placed.
3.1. Headway Constraints
First plot the positions of the running signals on the principal running lines. Then proceed to lower priority lines in order of importance.
It is often difficult to decide where to start. If a station exists on the layout it is usual to start by plotting platform starting signals, and then continue by plotting the signals in rear and in advance to be within the tolerance of minimum separation (for braking distance) and maximum separation (for headway) for the type of signalling to be employed.
It is also desirable to position signals close to facing points at junctions so that the driver does not have a long distance to travel to the turnout. This will minimise delay to traffic and reduce the possibility of signals being misread.
Service braking distance must be provided from the first cautionary aspect to the signal at danger in every case.
The maximum distance from the first caution to the red is set by the headway requirement. Ensure that any large excess over service braking distance is within acceptable limits.
3.2. Other Constraints
In addition to headways, other constraints must be borne in mind. These include junctions, further stations, tunnels, viaducts and level crossings.
There may be many places where it is required to stop a train for operating reasons. The engineer will have very little choice in the position of these signals. Conversely there are also many places where it is highly undesirable to stop a train and signals must be positioned to avoid these.
Other factors to consider are the visibility of the signal to the driver, the practicality of installing a signal at a particular site and ease of access for maintenance.
Signal A has been placed at a distance greater than service braking distance from B because it would have otherwise stood on the points 201.
If a line is generally signalled with 2 or 3 aspect signals and it seems necessary to have two signals spaced closer together than service braking distance, remember that the next signal in rear must be capable of displaying a medium aspect to give an earlier warning of the need to stop. If a suitable signal does not already exist, an additional signal will have to be introduced.
A signal should be positioned so that a train stopped at that signal does not:
- Stand in a tunnel (wholly or partially). This would not apply to underground passenger railways where special provision has been made in the design of the trains and/or tunnels to ensure the safe evacuation of passengers in an emergency.
- Stand on a viaduct, unless special provision has been made for safe evacuation of passengers and/or access by emergency services.
- Foul a junction.
- Stand partially at a platform (unless the passenger doors can be kept closed by the train crew).
There are occasions when, due to local circumstances, these requirements cannot be wholly met but every effort should be made to comply.
Additional considerations will apply on electrified lines to ensure that trains are not brought to a stand in neutral sections or gaps in the conductor rail.
If possible, heavy freight trains should not be stopped on steep falling or rising gradients, especially in combination with sharp curves.
3.3. Examples of Standage Constraints
-
- Ensure maximum length train can stand in platform.
- Ensure that maximum length train can stand clear of junction fouling point to allow other movements to pass behind it.
- Signals on adjacent running lines should as far as possible be placed opposite each other. This minimises the chance of drivers reading the wrong signal. It also simplifies the design and installation of power supply and location equipment.
4. TYPES OF SIGNAL
Running signals may be divided into three general groups according to the form of control exercised by the signalman.
4.1. Automatic Signals
Automatic signals are designed to operate only according to the presence of trains on the track circuits ahead. The signalman does not have to set the route for each train. Usually he will not have any facility to set routes but may in certain circumstances be provided with a switch or button to replace the signal to danger in emergency.
A signal may be shown as automatic if all the following conditions can be met:-
- No points in the route to the next signal.
- No points in the overlap beyond the next signal (Exceptionally, simple facing points may be allowed).
- No directly opposing routes within the route or the overlap.
- No ground frames, controlled level crossings or other equipment with which the signal must be interlocked.
The normal aspect of an automatic signal (i.e. the aspect shown when there are no trains present) will generally be a proceed aspect.
As the signalman is not directly concerned with the operation of automatic signals, some other facility may be required to stop trains in an emergency. If it is decided that this facility is required, options available are:-
- Individual replacement switches or buttons on the signalman's panel (for all automatic signals or selected signals only)
- Grouped replacement switches or buttons. Each one can replace a group of consecutive signals on the same line to danger)
- A replacement switch mounted on or near the signal, usually requiring a special key to operate.
Earlier British practice was to provide replacement facilities on automatic signals in any of the following circumstances:-
- Controlling the entrance to a section in which a level crossing equipped for automatic operation is situated.
- Controlling the entrance to a section in which a tunnel or viaduct is situated.
- Controlling the entrance to a section in which an electrical traction break is situated.
- If not otherwise required, on at least every fifth signal in any section of automatic signals and at any other signal where required for operating purposes.
Since the accident at Clapham Junction in 1988, this policy has now changed to one of providing some form of emergency replacement for all automatic signals. On all new installations, this is by means of an individual button on the panel.
Automatic signals must be recognisable as such to the driver. In the event of a signal failure and loss of communication with the signal box at the same time, the driver can then pass the signal at danger and proceed at extreme caution to the next signal (prepared to stop short of any obstruction).
Some railways identify their automatic signals by a different style or colour of identification plate. SRA (TfNSW) practice is to offset the lower signal lights (or marker light) 204mm to the right of the upper signal lights. If clearance constraints make this impossible, and the lights must be in line, a separate plate with a white letter A on a black background is provided.
4.2. Semi-Automatic Signals
In many areas signal boxes or local ground frames are provided which provide access to sidings or are not continuously manned. Signals must be provided which permit through traffic to operate when the frame or signal box is unmanned.
These signals have to operate automatically for much of the time but must also be capable of control by the local operator.
Semi-automatic signals are provided in this case. When the signal box is switched out or the local frame is locked normal, the signal functions as an automatic signal.
Once again, the driver must be aware of the correct action to take in the event of failure. A distinct identification plate is used on some railways. The driver must then confirm that the ground frame or local signal box is not in use before treating it as an automatic signal.
SRA (TfNSW) practice is to provide an internally illuminated "A" indication below the semi-automatic signal (which otherwise has the same appearance as a controlled signal). The "A" is illuminated only when the signal is working automatically.
It may also be necessary to divert any signal post telephone circuit to another supervising signal box when the local control is not in use.
4.3. Controlled Signals
Any signal other than those described above will be a controlled signal. It must be controlled from a signal-box (other than by Emergency Replacement). It will usually require a lever, switch, button, key or plunger to be operated for each movement.
Controls may be provided to allow a controlled signal to operate automatically (i.e. without re-setting the route for each movement). It must be decided whether this should be apparent to the driver. BR practice is not to provide any distinct identification on the signal. The driver will always treat it as a controlled signal, even when working automatically. This is because it is unsafe to adopt any form of "stop and proceed" working where there are points ahead of the signal which may be moved.
A signal will normally be "Controlled" if there are points and/or conflicting routes in the route or overlap.
4.4. Signal Identification Plates
All stop signals and/or all signals provided with a signal post telephone should be provided with an identification plate. As stated earlier this may be of a different style according to the type of signal (controlled, automatic or semi-automatic).
When talking to the signalman, the driver should be able to identify where he is, even if this information is also shown on the signalman's telephone equipment.
Other signals (e.g. distants and repeaters) may also be identified for maintenance requirements.
Some controlled signals in particular positions may need to be specially identified. For example, the "accepting" signal on approaching an interlocking from a section of automatic signalling is specially plated to remind the driver that he is no longer under the control of automatic signals.
4.5. Signal Aspects
Having decided on the necessary position of each running signal on our plan, we must now ensure that we depict each signal to show the correct combination of signal aspects.
This will be governed by two main factors:-
- the type and distance of the next signals ahead; whether it is necessary to give warning to the driver to stop.
- whether the signal or the next signal ahead is a junction signal. Any signal that has more than one running route is a junction signal and, as such, must provide the driver with the appropriate turnout aspect and/or route indication when required.
For all junction signals on the signalling plan, an adjacent box should be included showing the signal number, description of each route, its exit signal, and the route indication (if any) displayed.
4.5.1. Aspects for Through Running (Single light)
In single light signalling areas a stop signal must at minimum have a head with a red and a green light, with a marker light below.
If the next signal ahead is a stop signal at greater than braking distance, a caution aspect must be provided.
If the signal ahead is at less than braking distance and the following signal section is also shorter than braking distance, the signal must display also a medium (pulsating yellow) aspect. The signalling plan will show a letter "P" against the signal to indicate that it displays a pulsating yellow aspect. If there is braking distance between the next two signals, the medium aspect is not required.
Single light signal heads capable of showing only two lights should be shown on the signalling plan with the actual colours required (R for red, Y for yellow and G for green).
Distant signals will usually display caution and clear only. If there is inadequate braking between the next two adjacent stop signals ahead, a medium aspect (pulsating yellow) will also be required.
4.5.2. Aspects for Through Running (Double light)
All double light signals have two signal heads, one below the other. The top head is the stop signal and the lower head is the distant for the signal(s) ahead.
If the signal is not a junction signal, the upper head will be red & green only. The form of the lower head will depend on whether three or four aspects are required. The, rules are exactly the same as for double light but the aspects are different.
The lower (distant) signal head will display red and green only for 3 aspect signalling, red,yellow and green for 4 aspect.
The provision of low speed aspects is dealt with elsewhere in these notes.
4.5.3. Junction Signalling (Single Light)
The indication of a route diverging from the main line will be by either a turnout signal (maximum of one route to left and/or right) or a multi—lamp route indicator in conjunction with the main aspect (more than one route to left or right).
When the turnout signal is used, the main signal remains at red. All turnout signals must be capable of displaying a caution (three steady yellow lights) and may display a medium turnout aspect (three pulsating yellow lights) when the signal ahead is showing a proceed aspect.
When a route indicator is used, the main signal will display a steady or pulsating yellow (according to the next aspect ahead) and the appropriate route indication will be displayed.
Clear signals are not given through turnouts.
4.5.4. Junction Signalling (Double Light)
The
indication of a turnout on a double
light signal is by a yellow in the
upper signal head.
All junction
signals therefore require an upper
signal head with red, yellow and
green lights.
The lower signal will still indicate the state of the signal ahead, red if the first signal ahead past the junction is at stop, yellow if the next signal displays a proceed aspect.
If a junction signal is set for the turnout (either caution or medium) the previous signal will display a medium aspect, never a clear. Route indicators may be used where two or more turnout routes exist.
4.5.5 Low Speed Signals
Section 2.5 has dealt with the main situations in which low-speed signals are used. Firstly, check whether a conditionally cleared caution could adequately fulfil the operating requirements (the overlap should be at least 100m for a conditional caution). Low speed signals are not necessary for normal through running. They should, however be considered in cases where reduced overlaps can aid the regulation of traffic and/or headway for stopping trains and a conditional caution is not appropriate.
It is also recommended to provide low speed signals through any area where a track circuit would otherwise control three or more running signals. This is to localise the effect of failures by restricting the number of handsignalmen required in the event of track circuit failures.
4.6 Shunting and Subsidiary Signals
Having catered for all running moves, we must now provide for shunting and other non-running movements. Before starting to place signals on the plan, make sure you know exactly what movements are required.
Any movements which are not signalled will have to be authorised by handsignals. Handsignalled movements on lines where the majority of trains are properly signalled are disruptive to normal traffic and allow the possibility of human error.
Conversely, signals provided for movements which are never used are an additional and unnecessary initial cost to the project. They also represent a continuing maintenance cost and a potential source of additional failures.
Although the terms are often used interchangeably, there is a distinction between shunting and subsidiary signals. Subsidiary signals are part of a main running signal. Shunting signals are independent.
Subsidiary signals therefore only need a proceed aspect - the main signal provides the stop aspect. Shunting signals must display both stop and proceed aspects.
Subsidiary signals can broadly be divided into the following functions:-
- To shunt from a running line (in the normal direction of traffic) into a siding.
- To move forward from a running signal into an occupied section. A main route to the same destination may already exist. The provision of both main and shunt routes could assist operations in critical areas during track circuit failures.
- On a multiple track line, to shunt on to another line in the opposite direction to normal traffic.
- To move forward to a shunt signal facing the normal direction of traffic.
- On absolute block lines, to permit a train to pass the starting signal at danger for shunting purposes only. The shunting movement must return behind the starting signal
Unless the movement is on to a section of line which is not fully signalled there will need to be an exit signal to limit the extent of the movement.
Independent shunting signals can broadly be divided into the following functions:-
- To shunt between running lines from a position where no main signal is provided.
- To enter, leave or shunt between sidings.
- To shunt in the opposite direction to normal traffic.
- To limit the extent of any shunting movement (including "shunting limit" boards).
The diagram below shows examples of some common applications of shunting and subsidiary signals.
4.6.1. Calling-on and Subsidiary Shunting Signals
This will be provided where a movement must be authorised to pass a main signal at danger to enter a section which is or may be occupied. An example would be the coupling of two portions of a train in a station platform. Signal 4 on the diagram has a subsidiary provided for this purpose.
The usual aspect displayed is a miniature yellow. Some older double light signals display an internally illuminated "CO".
4.6.2. Dead End Signal
This will be provided where a movement must be authorised to pass a main signal at danger to enter a dead end siding via a facing turnout from the main line. It displays a miniature yellow light and is offset to one side of the signal post (according to the direction of the movement). Signal 5 is an example of this.
4.6.3. Shunt Ahead Signal
Used to shunt ahead of the starting signal and mounted below the main signal. Used on single light signalling only and displays a pulsating miniature yellow light. Signal 7 is provided with a shunt ahead signal to enable long trains to draw forward past the signal before shunting back over the crossover. This type of signal will normally be found in single light signalling areas only.
4.6.4. Dwarf and Position Light Shunt Signals
Shunting signals normally have two aspects - stop and proceed. The stop aspect is two red lights and the proceed aspect is a single yellow light. This instructs the driver to proceed at caution. It does not guarantee that the line ahead is clear.
SRA (TfNSW) uses both dwarf and position light shunting signals. The difference between the two types of signal is the orientation of the lights. The choice of signal type will depend mainly on lineside clearances. On a position light signal, the two red lights are side by side, the yellow light is above. A dwarf signal has the three lights vertically arranged; the red lights are at the top and bottom with the yellow light between.
Route indications are provided where required, particularly where wrong line movements are signalled.
4.6.5. Shunting Limit Boards
Effectively a shunting signal fixed at danger, a shunting limit signal faces in the opposite direction to normal traffic and is used to limit the extent of a wrong line shunting movement. An example is shown on the down line. This would enable trains to shunt out of No. 1 siding on to the down line before proceeding forward. Without the board there would be no signal to prevent the wrong line movement continuing indefinitely on the down line.
4.6.6. Facing (or Preset) Shunt Signals
Occasionally, shunting movements in the normal direction may be required to start from a position where a running signal is not provided. Such a shunt signal must therefore be passed by normal running movements. To avoid the driver seeing a yellow light after he has just passed a main signal showing clear (and possibly braking unnecessarily) "facing" shunt signals are provided with an additional green light to show clear when the previous main route is set past the shunt signal and the signal is showing clear. Signal 55 is a facing shunt signal.
4.6.7 Point Indicators
Point indicators should be provided on any points (whether facing, trailing or catch points) where the driver is responsible for observing the position of the points before proceeding over them. The points will usually be hand worked, as shown in siding 1 on the example.
Where regular shunting takes place without the need for the signalman to set the route for every move, point indicators will be displayed. These will be selected by a separate button on the signalman's panel. This is preferred to providing two shunt signals with opposing locking removed as it avoids the possibility of two trains approaching each other both under proceed aspects.
4.7 Trainstops
SRA (TfNSW) provides trainstops on most of the Sydney metropolitan area. Double light signalling is normally provided. All electric multiple unit trains are provided with tripcock equipment which will apply the brakes if a train passes a raised trainstop. The trainstops are provided at each main stop signal and in certain other locations (e.g. exits from depots and sidings) to prevent a rear end collision with another train.
If the signal is at danger, the trainstop will be raised. A train irregularly passing a signal at danger will be tripped and brought to a stand within the length of the overlap.
Where trainstops are to be used, the engineer must ensure that the length of each overlap is adequate for emergency braking at the highest speed at which a train is likely to pass the signal.
Obviously the trainstop cannot ensure total safety if all trains are not fitted but it can make a major contribution to safety in areas where trains regularly run at close headways.
An important part of the preparation of the signalling plan is therefore to decide where trainstops are to be positioned. This is closely associated with the calculation of overlaps. It may often be more important to accurately position the overlap for track circuit clearance purposes, then work back to the position of the signal and the trainstop.
A low speed signal tells the driver that there is little or no overlap beyond the exit signal. Running speeds will be low (normally less than 35km/h). The low speed overlap will be based on the passing speed of the low speed signal. However, the driver could fail to brake, or even accelerate after he has passed the low speed signal. This would leave an inadequate low speed overlap. Intermediate trainstops are therefore often provided between a low speed signal and the next signal, to be lowered only after sufficient time has elapsed for the train to have reached the trainstop at or below the correct speed.
The following general rules therefore apply to the positioning of trainstops:-
- A trainstop is required at all stop signals. It must always be on the same side of the line. SRA (TfNSW) provides trainstops on the left hand side, London Underground and British Rail use the right hand side of the track.
- Additional trainstops may be required on the approach to stop signals with a reduced overlap where a speed reduction has already been enforced at a previous signal. As an example, a low speed signal reading into a station platform could have a low speed aspect to allow early entry of following trains. The overlap associated with this may be 100 metres or less, even reducing almost to zero. To ensure that a train does not accelerate to a speed which would render the overlap inadequate, an additional trainstop is provided on the approach to the exit signal after the low speed signal. The lowering of this trainstop is timed to trip a train which is running above the permitted speed.
The positioning of trainstops therefore has to take account of the braking and acceleration characteristics of the train and the length of the overlap. The calculation can become very complex so a simple example is used here to illustrate the possibilities.
In the following diagram, the two stop signals are 200 metres apart. For headway and/or junction clearance purposes, it has been decided that only a 50 metre overlap is available beyond the second signal. We will assume that the train passes the first signal, displaying a low speed aspect at 27 km/h or less (otherwise it would have been tripped).
This example will assume a typical service braking rate of 0.9 m/s2, an emergency braking rate of 1.4 m/s2 and an acceleration rate of 0.55m/s2. These are typical of those which have been used for SRA (TfNSW) signalling for electric multiple units although the actual performance of the trains which will use a line must always be confirmed, and gradients taken into consideration.
The train should under normal circumstances brake to a stand at signal 2 along or below curve A. The trainstops should ensure that the train will come to a stand within the overlap, should the driver fail to take the correct action to control his train.
There are various possibilities which may arise. The signal engineer must decide whether to allow fully for all of these or whether circumstances will permit some relaxation.
- After passing signal 1 at the permitted speed (27 km/h in this example) the driver could totally fail to brake. Even worse, he could accelerate after passing signal 1. We could assume either no acceleration, acceleration due to gradient only or acceleration under full power. Whichever is chosen, the overlap should be greater than the emergency braking distance from signal 2. If this is not the case, an intermediate trainstop (labelled ITS) must be provided which should be timed to lower just before the train reaches it on a normal service braking curve (point X on the diagram). Curve D shows the effect of this trainstop on a train accelerating under full power.
- With the intermediate trainstop having been passed at the correct speed (lowered before the arrival of the train), the train could then accelerate at full power towards signal 2. In this case the trainstop at the signal will ensure the train stops within the overlap. Curve B shows the likely speed profile of the train in this situation.
- Even with these safeguards it is possible that a train could stand just past signal 1 on the timing track circuit for the intermediate trainstop. The trainstop would lower after the prescribed time interval and the train could then accelerate under full power towards signal 2 without the protection of the intermediate trainstop. It will be seen from curve C that the train will overshoot the overlap, passing the overlap joint at up to 9 m/s. To overcome this, the length of the timing track circuit for the intermediate trainstop must be limited such that an accelerating train could pass signal 2 at a speed no higher than that possible on curve B. Alternatively, an additional intermediate trainstop could be provided to check the train speed at an earlier point.
SRA (TfNSW) practice in open (i.e. above ground) areas is to allow some margin for possible acceleration but not the deliberate full acceleration of curve C. In tunnel sections where the driver's perception of speed and distance may be affected, the positioning of trainstops and their associated timing track circuits should cater for all possibilities. It should be noted that due to the introduction of newer trains with better acceleration characteristics, the protection provided by certain older sections of signalling is now reduced. It will still protect against most normal occurences other than the deliberately malicious driver intent on overriding the protection of the signalling equipment.
Typical distances for open areas are as follows:-
For following trains and overlaps less than 50 metres, the intermediate trainstop is positioned 100 metres from the end of the overlap with a timing track circuit between 80 and 220 metres in length.
For overlaps clear of fouling movements, the overlap should be at least 100 metres and the intermediate trainstop 200 metres from the fouling point. In addition, any previous signal whose full overlap extends beyond the fouling point should be conditionally cleared to caution.
This arrangement is not recommended where signal spacing exceeds 500 metres.
TO BE CONTINUED - SIGNALLING BOOK | CHAPTER 3 | PART 3...........
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