CH3 | SIGNALLING A LAYOUT | PART 3
Signalling
CONTINUED FROM - SIGNALLING BOOK | CHAPTER 3 | PART 2
SIGNALLING BOOK | CHAPTER 3 | PART 3
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
5. POINTS AND CROSSINGS
Although, in general, the siting of points and crossings on an existing railway will be dictated by permanent way design considerations, it is left to the Signal Engineer to determine the operation of the points. Furthermore, the Signal Engineer may require additional trapping protection to be provided on occasions and such cases must be referred back to the Permanent Way Engineer or other responsible engineer.
In the case of combined track remodelling and resignalling projects, it is sometimes possible to provide simpler or improved signalling controls by minor alterations to the track layout. Close co-operation between the Signal Engineer and the Permanent Way Engineer is essential if the optimum results are to be achieved.
5.1. Position and Numbering of Points
Any set of points will be defined as lying in its Normal position for one route and its Reverse position for the other route. The Normal position of the points will be shown on the signalling plan as follows:-
A similar convention applies to switched diamond crossings, if used.
Points should be numbered in such a way that any point ends required to work simultaneously carry the same number. To localise failures, it is not advisable to number more than two ends to work together. In addition, Solid State Interlocking (SSI) equipment is normally only configured to operate single and double ended points, although in certain circumstances three ends can be accommodated.
For control purposes, each end has to be identified separately (A or B) but this may not need to be shown on the signalling plan. A convention must be determined for identifying A and B ends (e.g. A end nearest control centre or A end at lowest reference distance etc.) and strictly observed. On TfNSW network a down train will meet the A end first.
5.2 Ground Frames
Ground frames control infrequently used points, usually outside interlocking areas. Although referred to as ground frames, they may equally well be locally operated control panels.
In its most common form the ground frame consists of just 2 levers, the point lever and a release lever (which will also work the F.P.L. if the points are normally facing). Movements over the points during shunting are usually controlled by handsignal, although extra levers may be provided to control or slot signals which the train must pass during shunting. Note the use of separate releases where the ground frame controls more than one function.
Instead of providing an electric lock on the release lever, a separate key is electrically released when the signalman operates the release button. This key is then used to release the ground frame release lever. It remains captive until the ground frame is normalised and can then be returned to the instrument to give back the release.
5.3 Trapping Protection
It may be necessary to request trap points (normally known as catch points on TfNSW) to be provided at certain locations:
- At the exit from sidings, where they lead on to running lines, catch or trap points must be provided to prevent an unattended vehicle running away or a shunting movement overrunning and fouling the running line.
- Where a full overlap cannot be obtained and movements are required to closely approach a converging junction, catch or trap points leading away from the running line can be used as an overrun in place of the normal overlap.
- On railways where a distinction is made between passenger and non-passenger lines, trap points may be used where the non-passenger line joins the passenger line.
Where trap/catch points occur in track circuited lines many railways employ a track circuit interrupter to ensure a derailed vehicle which is still fouling the track, although not standing on the rails, remains detected. The track circuit interrupter is normally insulated from the rail on which it is mounted and bonded in series with the opposite rail.
6. TRACK CIRCUITS
Track circuits shall be provided in a manner which permits maximum flexibility with minimum expense and complexity.
6.1 Overlaps
Running signals should, in general, be provided with separate berth and overlap track circuits, the berth track circuit terminating immediately beyond each signal. This will ensure the signal is replaced to danger at the earliest opportunity after the train passes. Where more than one overlap is required, a joint must be provided at the end of each overlap.
Calculation of overlaps has already been covered in the earlier sections dealing with the positioning of signals and trainstops. However, old TfNSW practice on overlaps is summarised below.
6.1.1. Where Trainstops Are Not Fitted
The overlap is a margin to allow for braking errors. There is no positive means of stopping the train if a driver completely misses or misreads a signal. As such it is an approximate distance based on experience, rather than one which has been calculated on any scientific principle. The standard SRA overlap is 500 metres. This may be smaller or greater than the actual braking distance. Where speeds are low, this is sometimes reduced. Recommended overlap lengths are:-
6.1.2 Sydney Metropolitan Area (Open Sections)
If trainstops are fitted,(recently ATP rollout has taken place which will remove the significance of mechanical trainstop) the overlap must be based on emergency braking distance for the prevailing speeds and gradients.
The following example shows how this may be calculated. To simplify the calculation when dealing with gradients it is often easier to express the braking rate as a percentage of the acceleration due to gravity (g = 9.8m/s2).
Braking distance = v2/2a (where a = braking rate, v = train speed)
= 100v2/2g(%B + %G)
%B = Braking rate as a percentage of g
%G = Gradient as a percentage (down gradients negative)
If the line speed is 25 m/s, the braking rate is 10% and the gradient is 1% down, the emergency braking distance and hence the overlap will be:-
100 x 252 / 2 x 9.8 x (10 - 1) = 62500 / 176.4
This would give a minimum overlap of 354 metres. This would probably have to be increased by a suitable margin to allow for less than 100% braking performance (e.g. some brakes isolated, wet or greasy rails, delay time for brake application).
Where available, braking tables or curves should be used.
If the full overlap is foul of junctions or station platforms, a reduced overlap should be considered. The train speed would have to be suitably reduced by a low speed or conditional caution aspect at the previous signal.
6.2 For Points and Crossings
The positioning of track circuit joints to prove clearance will depend on the dimensions of the rolling stock in use. One must first determine the difference between the maximum vehicle width and the width of the widest vehicle in service. The position must then be found where the rails leading away from the crossing are at least this distance apart (normally adding a small safety margin). At this point, the extreme ends of vehicles on the adjacent tracks will not be foul of each other.
Measuring away from this position, the joint must be located at a distance greater than the maximum end overhang of any vehicle. This is obtained by measuring from the centre of the outer axle to the extreme end of the vehicle.
Track circuits should allow maximum flexibility of use of the layout. In particular, where the track layout permits parallel moves, the signalling must not prevent them. Joints should be positioned to achieve the earliest release of points after the passage of a train consistent with safety, economy and practicality of installation.
Example 1
Joint A allows simultaneous moves over both ends of crossover normal.
Joint B allows points to be moved as soon as train leaves points.
Joints C, at clearance point, allow movements across crossover with Tracks X and Y occupied. It is common on plans to place joints C opposite the tips of the points.
Example 2
Joints A allows parallel moves.
Joints B allow points to be freed as soon as junction cleared.
Joints C are set back at clearance point. These may also be the overlap joints for signals approaching the junction.
Joint D will be dependent on factors other than the requirements for operation of the junction, eg. the position of the protecting signal.
7. NUMBERING OF SIGNALS, POINTS AND TRACK CIRCUITS
To enable all signalling controls to be specified, each signalling function must be uniquely identified. It aids design, testing and fault location if this is done in a logical and orderly manner. In particular, confusion is avoided if different types of functions are numbered in different number or letter series.
The main functions which need to be numbered are:-
- Main Signals
- Shunt Signals
- Points
- Track Circuits
- Ground Frame & other releases
Separate number series should be provided for each type of function (points, signals etc.). Main and shunt signals may be numbered in the same or separate series.
Lines for each direction of traffic are normally designated UP and DOWN. Signals reading in the Down direction normally carry odd numbers with the lowest number at the Up end of the control area. Signals reading in the Up direction normally carry even numbers, again with the lowest number at the Up end.
Points will be numbered with the lowest number at the Up end of the control area.
Where possible suitable gaps should be left in the numbering sequences in anticipation of future alteration. Distinct branches should be numbered in separate series.
Historically, several different conventions have been used for identification of track circuits. Each has advantages and disadvantages. One common method is to use a simple numbering sequence. The disadvantage of numbers is that, on a large installation, very large numbers or duplicate number sequences need to be used (with greater risk of errors in design and testing). Another alternative which has been used is to number track circuits based on the distance along the line. This results in track circuits in one locality having long and very similar numbers. Again confusion and errors may result.
TfNSW uses a system based on the signal numbers. The first track past signal 5 would be 5A, the next 5B and so on. The suffix E is not normally used.
The BR standard is now to use letters. Each track circuit indicated to the signalman should be identified using two capital letters, arranged alphabetically in a logical sequence. Letters I and O are not used. Where a number of track circuit sections have a common indication they should have the same identity plus an individual suffix number, eg. AA1, AA2 etc. This arrangement is simple but does not give any indication of the relative locations of tracks and signals.
8. EXAMPLES
A few examples are now given of some of the more commonly found track layouts and suggested arrangement of signals. They do not cover all situations. In practice, different requirements will conflict. The signal engineer must resolve these conflicts in the most effective and economic manner
8.1 Junctions
The logical arrangement at a junction is for the protecting signal to be as close to the junction as possible. For diverging movements this ensures that trains are not checked too far from the junction, while for converging movements it reduces the chances of trains being checked due to conflicting moves on the junction.
The signal next in rear of the junction cannot be cleared unless the section is clear up to the junction signal, and the overlap beyond. It is preferable that the overlap is not fouled by conflicting moves, so ideally the signals protecting the junction should be placed overlap distance in rear of the junction. If large overlap distances make this impractical, a reduced overlap clear of the junction should be considered. In this case the signal in rear should have its full overlap clear of the junction.
8.2 Station Platform with Loop
It is usually desirable for headway reasons to site a signal at the end of the platform. This provides a platform starting signal and also protection for any level crossing at this point. In situations where there is no platform starting signal, there is a risk that a station stop will divert the driver's attention sufficiently for him to forget the aspect displayed by the previous signal. After restarting his train, he could approach the next signal at an unsafe speed.
In the above example, signal A is located at full S.B.D. from signals B & C. For main line running this is satisfactory, but for a move into the loop the time taken for the train to slow to 25 km/h over the crossover may adversely affect the headways. A better arrangement is shown below :
Signal A is moved closer to the turnout. If this results in inadequate braking distance from signal A to signals B & C, signal D must be a 4 Aspect signal. Signal A would only need to be 4 aspect if the signal ahead of C was less than braking distance.
8.3 Terminal Stations
The station throat track
capacity must be at least double
that of the approaching line. This
is because some arriving and
departing movements will completely
block all other routes.
The signal reading into the platforms should be as close as possible to the plaforms. If this signal does not have an overlap clear of points, the signal in rear should do so to allow trains to approach unrestricted.
Signal spacing on the approach to the station should be as close as possible consistent with standing room and headway requirements.
The first signal leaving the station should be as close as possible (whilst retaining necessary train standage clear of the pointwork), to allow the best possible aspect on the platform starting signals. The standage requirement may have to be reduced to maintain adequate line capacity on the departing line.
The platform entry signal 3 exhibits stop and caution aspects only for running moves. Buffer stops are equivalent to a signal permanently at stop.
Subsidiary shunt signals are provided to enable trains or locomotives to enter occupied platforms.
If wrong line shunting is required, a shunt limit board must be provided at a position which permits adequate standing.
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