Signalling and Interlocking


Introduction

Signalling and Interlocking is a topic which is often shrouded in mystery for the modeller, primarily because a reasonable amount of the necessary information is not generally published in the public domain.

In this article, we will show a worked example of how to signal and interlock a sample layout using prototype rules and regulations. We will use the following fictitious station called 'Harburn':

Harburn Raison D'etre

Harburn is an intermediate station on a single track branch line. It has two platforms and therefore provides a passing loop for trains travelling in opposite directions. London (Up) is on the right end of the layout. In the winter months, the Down platform is closed to passenger services and no passing loop is operated - the operating timetable is reduced such that there is no need for passing of passenger trains. The Up platform is therefore, bi-directionally signalled. There is a small goods yard on the up side.
Semaphore signalling is in operation as this layout represents steam operation in the 1960's. The signalling principals applied are the same today as they were then for the purposes of this article.


A few Definitions of Terminology

The following terms will be used in this article:

TermMeaning
TurnoutThe Permanent Way Engineer's term for what modellers call a 'point' or 'set of points'
RouteConventionally assumed by modellers to mean the 'route' from London to Scotland. In prototype terms, a route is the path between one signal and the next. It can be a short length of track and/or can contain a number of turnouts
MASAn abbreviation for Multiple Aspect Signalling. (colour light signalling)
OverlapA length of track beyond a signal in which a train can stop if it accidentally passes a signal at danger. There are rules on how long an overlap must be and this is based on a number of factors including, but not limited to, line speed, train braking type, gradient and signalling type (semaphore vs MAS). For the purposes of this article, all prototype overlaps are far longer than is practical to model, therefore, we will assume that we will use the minimum length permissible which in 4mm scale scales down to approximately 26 inches. This actually represents all parameters at their minima i.e. walking pace speed etc
'Pulled off'A railway term meaning a signal positioned at green (go)
'Signal at Danger'A signal positioned at red (stop)
NormalThe regular position a turnout is left in. For most turnouts, this is normally the 'straight' position, however, it need not necessarily be so
ReverseThe opposite of 'Normal'
SPADSignal Passed At Danger (Run a Red Light)
IECCIntegrated Electronic Control Centre - a modern computer control centre which usually operates an entire regional area


Defining the Operation

Before we can signal a layout, we need to know what its operation will be - how it is intended to be used.
Prototype practice is for signalling and permanent way engineers to work in cooperation with the operating departments so that a layout is designed to provide the movements required and which complies with all the design rules and regulations of permanent way and signalling, typically laid down by HM Railway Inspectorate.

To that end, the track layout has been designed with this knowledge.

Harburn is on a single track passenger line. Up trains pass through the Up platform and Down trains pass through the Down platform. In winter, the Down platform is closed and all trains pass through the Up platform.

There is a small goods operation which is conducted in the goods sidings. Typically, this serves a coal merchant and a general dock for van loads of goods.

Goods trains travelling up to London stop on the main running line and back down into the goods yard to collect local goods and produce to take to London.

Goods trains travelling Down are dropping empty wagons off or goods and produce sent from London.

These trains stop in the Down platform, the loco runs round the train and takes wagons off the back of the train and moves them to the goods yard. The loco re-couples to the front of the train and then continues Down.

There are therefore, goods trains in both directions all year round.


Defining the Operational Rules

There are a few rules which must be applied to this layout:

  • Because this is a passenger railway, signal overlaps are required
  • Where overlaps are too short, trains must be held at previous signals (discussed later)
  • The passenger service must be protected from the goods service
  • All operational movements on the passenger running lines must be signalled


Pass 1: Analysing the Interlocking

The process of interlocking and signalling a layout is a multi-pass, iterative process.

The first thing we need to do is number all of our turnouts. Turnouts which operate as a crossover pair are given the same number and a suffix of A and B. By convention, B is always furthest from the controlling signal box.

Please see the diagram above for the turnout numbering.

We now look at each turnout in turn and determine the rules by which they can operate and determine the signalling necessary for the operation across that turnout. As we identify signals, they will be numbered, typically using a lettered abbreviation of the signal box controlling the signal, followed by a number. The objective is to form a matrix of all the relationships between signals and turnouts.

The following matrix shows the relationships between all of the signals and turnouts and indicates when turnouts can or cannot be changed. Where there is no relationship, between a turnout and a signal, the turnout may be freely changed. Following the matrix, we will analyse each turnout in turn:

Turnout 1A
This turnout is on a passenger running line and must be protected by signals. As we identified in the operations above, trains pass over this turnout in both directions:

  • By trains passing through Harburn and heading up to London
  • By locos running round goods trains in the Up directions
  • By trains heading down from London during the winter service
  • By locos running round goods trains in the Down directions
This means we need an outer home signal which we will call HN1.1 and a starter signal which we will call HN2.

The interlocking rules which apply around Turnout 1A are:
  • The turnout is locked (cannot be changed) if either signal HN1.1, HN1.2 (see later), HN2 or HN3 (see later) is pulled 'off' (green)
  • All the rules of Turnout 1B since 1A and 1B operate as a pair
Turnout 1B
This turnout is on a passenger running line and must be protected by signals. The operations across this turnout are:
  • Up trains and locos running round goods trains pass over this turnout in Reverse position
  • Down trains pass over the turnout in Reverse position
  • Down trains pass over the turnout in Normal position
  • Locos running round goods trains in the Down direction pass over the turnout in Reverse position.
We already have signal HN1.1 to protect the turnout for Up trains, however, we need signal HN3 to protect it for Down trains.

The interlocking rules which apply around Turnout 1B are:
  • The turnout is locked (cannot be changed) if either signal HN1.1, HN1.2, HN2 or HN3 is pulled 'off' (green)
  • All the rules of Turnout 1A since 1A and 1B operate as a pair
In practice, this turnout would probably have a Facing Point Lock (FPL) with a bar, parallel to the running rail which is depressed when the flanges of a train pass over it. The lock prevents the turnout from being changed under a train.

Turnout 2A
This turnout is on a passenger running line and must be protected by signals. The operational movements across this turnout are:
  • Down passenger services
  • Down goods trains
  • Locos coupling up on the rear of goods trains in the Down platform to remove wagons
  • Reversing movements into the Down siding (typically goods wagons being dropped off)
We therefore need:
  • Ground shunt signal HN5.1 to permit a loco moving wagons from the Down platform back to the Limit of shunt beyond signals HN6
  • Ground shunt signal HN5.2 to permit a reverse shunt into the Down siding
  • Signal HN6.1 to permit a Down train to enter the Down platform
  • Signal HN8 to permit a train to leave the Down siding and enter the Down platform
The interlocking rules which apply around Turnout 2A are:
  • The turnout is locked (cannot be changed) if either signal HN5.1, HN5.2, HN6.1 or HN8 are pulled 'off' (green)
  • All the rules of Turnout 2B since 2A and 2B operate as a pair
Turnout 2B
This turnout is on a passenger running line and must be protected by signals. The operational movements across this turnout are:
  • Down passenger services (through either the Up or Down platform)
  • Down goods trains (always through the Down platform)
  • All Up trains
  • Locos coupling up on the rear of goods trains in the Down platform to remove wagons
We therefore need:
  • Signal HN4 to permit all trains to leave the Up platform for London
  • Ground shunt signal HN5.1 to permit a loco moving wagons from the Down platform back to the Limit of shunt beyond signals HN6
  • Ground shunt signal HN5.2 to permit a reverse shunt into the Down siding
  • Signal HN6.1 to permit a Down train to enter the Down platform
  • Signal HN6.2 to permit a Down train to enter the Up platform
The interlocking rules which apply around Turnout 2B are:
  • The turnout is locked (cannot be changed) if either signal HN5.1, HN5.2, HN6.1, HN6.2 or HN8 are pulled 'off' (green)
  • All the rules of Turnout 2A since 2A and 2B operate as a pair
Turnout 3A
This turnout is on the entry and exit to a goods yard. It is not on a passenger running line but because it enables trains to pass out onto a passenger running line, protecting signals must be provided. The operational movements across this turnout are:
  • Goods movements into the goods yard (Reverse position)
  • Goods movements out of the goods yard (Reverse position)
  • Shunting movements within the goods yard (Normal position, using shunt neck)
We therefore need:
  • Ground shunt signal HN7 to permit trains to exit the goods yard. This needs to be a yellow shunt signal because it can be passed at danger to move into the shunt neck when 3A is in Normal position
  • Signal HN6.3 to permit trains to enter the goods yard
The interlocking rules which apply around Turnout 2A are:
  • The turnout is locked (cannot be changed) if either signal HN5.1, HN4, HN5.1, HN6.1, HN6.2 or HN6.3 are pulled 'off' (green)
  • All the rules of Turnout 3B since 3A and 3B operate as a pair
Turnout 3B
This turnout is on a passenger running line and must be protected by signals. The operational movements across this turnout are:
  • All Down passenger trains (through either the Up or Down platform)
  • All Down goods trains (always through the Down platform)
  • All Up passenger trains
  • All Up goods trains
  • Locos coupling up on the rear of goods trains in the Down platform to remove wagons
  • Locos removing wagons from the rear of goods trains in the Down platform
We therefore need:
  • Signals HN6.1, HN6.2, HN6.3 to permit Down trains
  • Signal HN4 to permit Up trains
  • Signal HN5.1 to permit shunt movements
  • Signal HN7 to permit trains to leave the goods yard
The interlocking rules which apply around Turnout 2B are:
  • The turnout is locked (cannot be changed) if either signal HN6.1, HN6.2, HN6.3, HN4, HN5.1 or HN7 are pulled 'off' (green)
  • All the rules of Turnout 3A since 3A and 3B operate as a pair
In practice, this turnout would have a Facing Point Lock and bar to prevent it being changed under a train.

Turnout 4
This turnout is in the goods yard. Goods yards are not signalled with the exception of their entries and exits. Therefore, Turnout 4 would not be under signal box control and would be hand operated with a lever with no interlocking.


Pass 2: Analysing the Signalling Interlocking

Having identified all the logic relating to turnouts and some of the signalling logic, we need to undertake a second pass across the signals because the signals themselves are also interlocked with logic and we may have introduced relationships on the turnouts we looked at last which actually affect those we looked at first.

The objective is to complete our matrix above and build up a second matrix of relationships between signals:


The horizontal axis of the matrix represents all of our signals, whereas the vertical axis represents the signal relationships with each signal across the top.

Note that any signal can be placed at danger by the signalman at any time, regardless of the state of the layout and therefore, there is no interlocking applied on this process.

Each dependency is now described:

Signal HN1.1

  • Cannot be pulled off if HN2 is green
  • Cannot be pulled off if HN3 is green
  • Cannot be pulled off if HN6.1 is green because the overlap between HN4 and 2B is insufficient, therefore, we hold the train at HN1 (this rule is generally not known to modellers)
  • Cannot be pulled off if HN6.2 is green
Signal HN1.2
  • Cannot be pulled off if HN2 is green
  • Cannot be pulled off if HN3 is green
  • Cannot be pulled off if HN6.1 is green (risk of head on collision in Down platform)
  • Cannot be pulled off if HN8 is green (risk of head on collision in Down platform)
Signal HN2
  • Cannot be pulled off if HN1.1 is green
  • Cannot be pulled off if HN1.2 is green
  • Cannot be pulled off if HN3 is green
  • Cannot be pulled off if HN4 is green because this could potentially pull a train apart if there is a loco on both ends (this rule is generally not known to modellers)
Signal HN3
  • Cannot be pulled off if HN1.1 is green
  • Cannot be pulled off if HN1.2 is green
  • Cannot be pulled off if HN2 is green
  • Cannot be pulled off if HN5.1 is green (risk of pulling a train apart)
  • Cannot be pulled off if HN5.2 is green (risk of pulling a train apart)
Although the overlap from HN2 to 1A is short, HN3 is allowed to be pulled off even if HN6.2 is pulled off because if a Down train overruns HN2, it will not collide with the train leaving HN3 because 1A will be at normal position, therefore, a train overrunning HN2 will pass into the shunt neck (known as an 'infinite overlap').

Signal HN4
  • Cannot be pulled off if HN2 is green (risk of pulling a train apart)
  • Cannot be pulled off if HN5.1 is green
  • Cannot be pulled off if HN5.2 is green
  • Cannot be pulled off if HN6.1 is green
  • Cannot be pulled off if HN6.2 is green
  • Cannot be pulled off if HN6.3 is green
  • Cannot be pulled off if HN7 is green
Ground Signal HN5.1
  • Cannot be pulled off if HN3 is green (risk of pulling a train apart)
  • Cannot be pulled off if HN4 is green
  • Cannot be pulled off if HN5.2 is green (can't signal two moves from the same place as the same time!)
  • Cannot be pulled off if HN6.1 is green
  • Cannot be pulled off if HN6.2 is green
  • Cannot be pulled off if HN6.3 is green
  • Cannot be pulled off if HN7 is green
  • Cannot be pulled off if HN8 is green
Ground Signal HN5.2
  • Cannot be pulled off if HN3 is green (risk of pulling a train apart)
  • Cannot be pulled off if HN5.1 is green (can't signal two moves from the same place as the same time!)
  • Cannot be pulled off if HN6.1 is green
  • Cannot be pulled off if HN8 is green (risk of head on collision)
Signal HN6.1
  • Cannot be pulled off if HN1.1 is green (HN4 - 2B overlap too short)
  • Cannot be pulled off if HN1.2 is green (risk of head on collisions)
  • Cannot be pulled off if HN4 is green
  • Cannot be pulled off if HN5.1 is green
  • Cannot be pulled off if HN5.2 is green
  • Cannot be pulled off if HN6.2 is green (can't signal more than one move from the same place at the same time)
  • Cannot be pulled off if HN6.3 is green (can't signal more than one move from the same place at the same time)
  • Cannot be pulled off if HN7 is green
  • Cannot be pulled off if HN8 is green
Signal HN6.2
  • Cannot be pulled off if HN1.1 is green (risk of head-on collision in Up platform)
  • Cannot be pulled off if HN4 is green
  • Cannot be pulled off if HN5.1 is green
  • Cannot be pulled off if HN6.1 is green (can't signal more than one move from the same place at the same time)
  • Cannot be pulled off if HN6.3 is green (can't signal more than one move from the same place at the same time)
  • Cannot be pulled off if HN7 is green
Signal HN6.3
  • Cannot be pulled off if HN1.1 is green (HN4 to 3B overlap potentially too short)
  • Cannot be pulled off if HN4 is green
  • Cannot be pulled off if HN5.1 is green
  • Cannot be pulled off if HN6.1 is green (can't signal more than one move from the same place at the same time)
  • Cannot be pulled off if HN6.2 is green (can't signal more than one move from the same place at the same time)
  • Cannot be pulled off if HN7 is green
Shunt Signal HN7
  • Cannot be pulled off if HN1.1 is green (HN4 to 3B overlap potentially too short)
  • Cannot be pulled off if HN4 is green
  • Cannot be pulled off if HN5.1 is green
  • Cannot be pulled off if HN6.1 is green
  • Cannot be pulled off if HN6.2 is green
  • Cannot be pulled off if HN6.3 is green
Shunt Signal HN8
  • Cannot be pulled off if HN1.2 is green
  • Cannot be pulled off if HN5.1 is green
  • Cannot be pulled off if HN5.2 is green
  • Cannot be pulled off if HN6.1 is green
HN8 can be pulled off if HN1.1 is pulled off because the overlap from HN3 to 1B is sufficiently long to arrest a SPAD at HN3.


Signalling and Interlocking Implementation

Now that we have defined all of our interlocking logic and signalling requirements, we can implement them on the layout. Historically, modellers would have implemented interlocking using hard-wiring of electrical switch-gear, mechanical interlocking or a combination of the two. Today, we have the benefit of computers and software such as the SSI Model Railway Control System which mimics modern IECC systems.

The actual signals used on the layout would be as follows:


Signal HN1.1 / Shunt Signal HN1.2




This signal could take one of two forms. The first version uses a semaphore to permit a passenger train to run into the Up platform and a shunt signal to permit a loco shunt into the Down platform.

The second version shows two semaphores, the left and higher arm permitting 'normal route' running into the Up platform and the right arm permitting a shunt into the Down platform:


Signal HN2 / HN3 / HN4


Ground Shunt Signals HN5.1 / HN5.2

At Harburn, the top disc would be HN5.1 and the lower disc HN5.2 (top-most disc is left-most route and bottom-most disc is right-most route):



Bracket Signal HN6.1 / HN6.2 / HN6.3


Ground Shunt Signal HN7

When at 'danger' position (as shown in this photo) this signal can be passed in order to proceed into a shunt neck. When pulled off (rotated), the signal permits a route out of sidings onto a running line.



Ground Shunt Signal HN8

These pictures show both WR and SR versions of the same type of ground signal:





Entry/Exit Route Setting

Entry/Exit Route Setting is a method used to set routes (true railway meaning of the word).
It is normally only implemented in environments where electronic or pneumatic signalling and turnout motors are in operation and involves the signalman pressing a button on a control panel to set the Entry signal from where a movement is to start and then pressing another button on the panel to set the Exit (end) of the route. Different railway administrations had different physical implementations of these buttons, particularly between countries. Once the Exit is set, the electronic system controlling the signalling and turnouts proceeds to try set the route but it will only do so if all the interlocking conditions (as we defined above) are satisfied.

In a modern IECC, computer screens display a map of a layout (the samples used in this article come from our SSI software which mimics real IECC panels) and the signalman clicks with a mouse (usually a roller ball) on the Entry and Exit signals.

Our Harburn station represents 1960's period operation where Entry/Exit route setting was not in operation, however, if we were to 'modernise' the location and use MAS and/or local Solid State Interlock devices (SSIs), the following routes would be defined:

  • HN1.1 - HN4
  • HN1.2 - HN3 (for goods locos returning to the front of their trains)
  • HN2 - EX1 (EX1 is known as an 'isolated exit' which simply provides an Exit to click on. It doesn't represent a physical signal, however, in practice, a real signal may well be placed at EX1)
  • HN2 - HN1.x (for goods locos returning to the front of their trains)
  • HN3 - EX1
  • HN3 - HN1.x (for goods locos running round their trains)
  • HN4 - EX2 (see note on 'isolated exits' above)
  • HN4 - HN6.x (goods loco run rounds)
  • HN5.1 - HN6.x (goods loco run rounds)
  • HN5.2 - EX3
  • HN6.1 - HN3
  • HN6.2 - HN2
  • HN6.3 - EX4
  • HN7 - EX2
  • HN7 - HN6.x
  • HN8 - HN3
Note that when a route is configured, the data specifies all of the turnout positions and signal settings required for a route and is coded into the local SSI or similar management device. When a route setting is requested, the system checks the devices specified in the route and checks to see whether they can all be set as required. If not, the setting is denied. If they can, then all devices will be set according to the route data.

The GPP Software SSI Model Railway Control System software implements interlocking in this prototypical manner. Note that actual interlocking rules are specified against individual turnouts and signals. The route itself never has interlocking logic embedded, contrary to how some other model railway computer control software products are implemented.


Track Circuiting

Track circuiting is also an integrated part of interlocking. The presence (or not) of a train in a track circuit will affect whether another train is permitted to enter the track circuited section of track.

Track circuiting was implemented in steam days, although unless Harburn was on a branch which saw frequent traffic, it may not have had track circuiting.

The presence of track circuiting was denoted by a horizontal white diamond fixed to a signal post.

However, assuming track circuiting was to be added to Harburn, we need to identify where our trains are likely to stop. Typically, there would be track circuits outside HN1 and HN6.x (so that the signalman knows there is a train waiting to enter 'station limits') and both platforms would have track circuits.

The signalling rules we defined above would be extended, so for example, if we look at HN1.1, the rules would now be:

  • Cannot be pulled off if HN2 is green
  • Cannot be pulled off if HN3 is green
  • Cannot be pulled off if HN6.1 is green because the overlap between HN4 and 2B is insufficient, therefore, we hold the train at HN1 (this rule is generally not known to modellers)
  • Cannot be pulled off if HN6.2 is green
  • Cannot be pulled off if TR2 (a track circuit in the Up platform) is occupied


Summary

While this article is not an exhaustive study of signalling and interlocking, we hope that it goes a long way to helping provide railway modellers with an insight into what is a vast subject.

We hope that it has demonstrated the following concepts:

  • That signal engineering is a discipline which takes place concurrently and in cooperation with the disciplines of permanent way engineering and operations - the prototype does not follow the conventional modeller's practice of designing track, then working out what the operation and signalling should be - this is too late!
  • How turnouts are interlocked with signalling
  • How signals are interlocked with turnouts and other signals, including signals which may be at the opposite end of a layout
  • How track circuits integrate with signalling interlocking
  • The concept of overlaps
  • The rule that signals on both ends of a platform cannot be pulled off at the same time
  • The principals of Routes and Entry/Exit route setting


Graham Plowman (10/05/2019)



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