Clearing misconceptions about absolute block signalling.

Overview

This is a short guide explaining how absolute block signalling worked on the real railway. I will cover the basic components from the driver’s perspective & the signalman’s perspective. I will also cover signals not yet available in train simulator but add to the operation of Britain’s railways in the steam era.

The problem:

I have written this guide because a number of routes in trainsimulator seem to confuse track circuit block system (TCB) with the Absolute block signalling. One route which suffers from this misconception is the Riviera line in the 50s. The crux of the issue is that while home signals can be scripted to behave as 2 aspect TCB signals, there are far too many distant signals for the route. in fact almost every home signal is paired with a distant signal. Even so 2 aspect colour light TCB signalling is almost nonexistent in real life as 2 aspect colour light signals usually form direct replacements for their absolute block counterparts. The only case I know of where full TCB was used on semaphores like the way that the riviera line in the 50s suggests, is the Liverpool overhead railway (see further down in the guide.)

A similar situation occurs with with the home / distant signal, where this has been confused with a 3 aspect TCB operated signal. While several home / distant signals could follow each other on really busy lines, (more on that later) they seem to be used far too liberally again, and in some cases they are used on the fast lines only.

All of this is unnecessary on the semaphore signalling system as unlike this setup, semaphore signalling is actually less restrictive on high speed running. In fact, under the right conditions, a driver can know about the state of the signals ahead for a greater distance than 4 aspect TCB operated signals can provide.

The issue gets worse when using signals scripted this way in the editor in the correct manner. If the signals did operate in the way suggested by this route and many others, then a very nasty collision would inevitably occur.

Absolute block signalling from the driver’s perspective.

First I will go through each semaphore signal currently in train simulator, and describe it’s role.

One thing to bear in mind is that from the 1930s onwards, all the big four companies (With the exception of the GWR) adopted the American upper quadrant signals where the arm is raised by 45 degrees for an off indication. The thinking behind this was that gravity would provide a fail safe return to danger if signal cable broke. The GWR and the former pre-grouping companies had Lower Quadrant signals. These were lowered by 45 degrees for an off aspect, and made use of heavy spectacle lens to bring the arm back to danger in the case of a fault.

Home signals Home signals are the simplest of all the semaphore signals, so I shall not dwell on this for too long.
Using 2 aspect MAS scripting affects the operation of a home signal the least as it can only show ‘on’ (danger and horizontal arm) or ‘off’ (clear to proceed to next home signal.)

Distant signal This is where the most trouble comes from the misconception, as a distant signal in real life is operationally almost an entirely different beast.

At the most simple, a distant signal shows that the next signal will also be on when it is on, or crucially when it is off ALL signals for the route set will also be off between it and the next distant. Distant signals scripted like the Riviera ones can only show what the next signal shows, which could easily mislead the driver into thinking that several signals after it are also clear.

When a distant signal is on, the driver should expect to see the next signal at on too but this isn’t always the case. When the driver reaches the home signal immediately after the distant signal and it clears, then the driver can expect all the other signals between it and the next distant signal to be off.

Distant signals could be permanently fixed in the on position in order to warn drivers that the next signal would likely be on, as well as on the approach to a terminus station or junction with significant speed restrictions in force.
It is worth noting that until the early 1920s distant signals would be painted red and would have a red caution lens when on, like a home signal.

Combined home & distant signal At busy locations, where the distant signal for the next section would not have enough braking distance, the distant signal may be placed on the last home signal of the section (more on this later). What the driver needs to know is that when the home & distant are both on, then the driver must stop. If the Home is off then the distant signal below can be read like any ordinary distant signal. If the home is on and the distant is off, then the signalman should be contacted as soon as possible as this should never happen!

Junction signal At an actual junction, or where a siding left the running lines, a junction signal indicates which route is set. It can either be arranged as a home signal like in the picture shown, or a combined home distant signal where there are multiple sections close together. When both signal arms are on, the driver must regard this as a home signal that is on. When one of the arms is off, then the route has been set for whichever the arm corresponds to. The arms are arranged so that the most important route will have the tallest arm. In this case the arm on the left is the most important route which is straight ahead, and the shorter post holding the right arm is on the diverging route.

Splitting distant signal This is quite similar to the junction signal, although it shows in advance whether or not the signals in the sections between the next distant are off for each route respectively, depending on the route set. This was a fairly uncommon signal, but the GCR & GNR and later the LNER favoured this arrangement on the approach to a junction. In this picture, it is arranged as a combined home distant signal, only clearing to show the route set ahead once the top home is off, and the section for the set route is clear.

Route Indicator Where there are many routes on a set of lines with a low speed limit, then a route indicator can be used to save space. This replaces many signal arms and shows which route is to be taken through the indicator when the home is off.

Siding signal This is a variant of the home signal used by the GWR. It was essentially a home signal, which controlled trains leaving a goods siding and rejoining the mainline. It should be read in the same way that a home signal would.

Ground signals Ground signals control shunting movements around a yard or station. The key difference is unlike a home signal, when off they indicate that shunting moments may proceed where the line is clear and not that the line is clear up to the next signal. Ground signals are typically lower quadrant, with the only exception being the LNER. Ground signals may also be painted with a miniature distant arm instead if shunting movements can pass the ground signal when on for a headshunt movement. It does not have anything to do with ground signals ahead unlike the full size distant signal. Where there are multiple sidings, the ground signals could be stacked. In this case the ground signals correspond to the lines from left to right reading from the top signal.

Absolute block signalling from the signalman’s perspective.

This section will focus more on how signals were placed and the operation behind them, rather than the individual functions which is described fully in the previous section.

The absolute blocks of the semaphore signalling system
The line is controlled by a number of signalboxes spanning the route; each controlling a small section of the route, from the distant signal leading into the section to the last ‘home’ signal before the next distant. Here each signalbox is responsible for not allowing any more than one train per absolute block.

The simple double line example shows 3 blocks controlled by signal boxes A, B & C respectively on the up side.

Labelling the signals & what is an absolute block?
You can see that in the diagram the two home signals controlled by each signalbox, labelled home & starter. While the driver can just regard these two signals as two home signals as there is no difference to the driver, the signalman would regard the signals as different because of their position along his section. Home signals are the first signal after the distant, and this may have the distant placed underneath as a combined signal if the adjacent box is to close for separate signals. The home signal protects any sidings upon entering the section and the last starter signal controls the exit of a train from the section. In some cases where there are multiple sidings running off the line in one block, there may be more than two signals, in which case the last starter would be called the advanced starter.

An absoulute block runs from the starter signal, through the next signalbox distant & home signal. In the absolute block signalling practice, no more than one train at any one time may enter the absolute block. Sharing this absolute block between two sections allows for both signalboxes to check the presence of the train. As a double measure, each train is also written into a traffic register; noting the class, destination, etc. This is important in absolute block signalling as the system was devised before the track circuit was devised, meaning that the bellcodes and block instruments between each signalbox were the only way to know where the train was in the section out of sight.

Bringing track circuits into the mix
While the absolute block signalling system relies on each signalman following the regulations surrounding the operation of the signalbox to the letter, this did not stop accidents from occuring. If the signalman mistook or forgot a train in the signalbox under the pure absolute block operation, then there would be no way of telling the signalman that there still is a train in his section. Beyond the driver informing the signalman of his presence (which later became mandatory under rule 55, if he was stuck at a signal at danger for too long.); a disaster such as the infamous incident at Quintinshill could happen. So, a method of reminding the signalman of any trains in his section was needed.

Enter track circuits:
The use of track circuits was devised by William Sykes (who would later become known for interlocking signals in his ‘lock & block’ system (see below) in 1864. The system uses a current passed through the rails which is shorted by any passing trains. This short switches on a light on in the relevant part of the signal panel diagram fitted in each signalbox. The signal panel diagram shows all the signals & points controlled by the signalbox for that section. This allows the signalman now to see where the train is, and to remind him of any trains (just in case he forgets on a long shift!)

Now that continual feedback of the presence of a train was available, it wasn’t long before new systems could do away with the old absolute block system. While absolute block signalboxes were being supplemented with track circuiting, the system could be adapted to control signals themselves. One of the earliest railways to do this was the Liverpool Overhead Railway.

in track circuit block operation, the block is now only between each signal. The track circuit relays to the signalman which ‘block’ the train is in, and depending on whether or not the track circuiting equipment automatically changes the signal (used on plain running lines typically), or manually the signal is changed to reflect how many blocks are free ahead. This is the standard modern signalling system that now sees huge panels controlling many miles of line. As the blocks exist between each signal, and there is no overlap between signal boxes (signalling centres nowadays usually) , there is no limit to the size of each TCB worked section.

Please see [link] for more on TCB operation.

Example operation.
In the following video, the operation of the 3 signalboxes is described.
It is important to note however, that the distant signal is ‘interlocked’ meaning that the distant signal cannot be pulled off if any signal in that section is on. The opposite is true when returning the signals to on, as no signals beyond the distant can be pulled back to on unless the distant has first been pulled to on. This is why it is impossible for the signalman to pull a distant signal to off on a combined signal if the home is on!
For more on bell codes, please refer to the appendix.

Automatic train control and Automatic warning system in absolute block signalling.

Unlike MAS signalling where an AWS ramp is provided at every signal, there is only a warning ramp at a distant, or combined distant signal. This is because there is no need under absolute block signalling to warn drivers about home signals, and it would only confuse drivers if such a system was in place. What is important to warn drivers about though is the state of the distant signal as that tells the driver what to expect in the next section. Here is an extract from the 1936 GWR general appendix showing what drivers were expected to know about ATC.

Appendix:

I) GWR standard signalbox bellcodes 1936 (please click on the picture for a closeup view)
II) Semaphore signals not currently in official trainsimulator content (X) / not currently in train simulator (N)

Subsidiary signals (N) Subsidiary signals were small additional arms attached to a home signal in order to give additional information to the driver held at the home signal. When the home signal is on, the subsidiary signal may be pulled off to show one of the 3 letters underneath:
C – calling on signal: This is the most common letter to be found on a subsidiary signal, which acted like a ground signal. When off the driver may proceed with caution knowing that the line ahead is blocked. This type was often found at busy stations to allow for empty coaching stock movements, and to allow multiple trains to join or split at a station.
S – shunt signal: like the calling on signal, but to allow for shunting movements not covered by ground signals.
W – warning signal: This was to indicate that the block section was clear up to a station or junction. This tells the driver to proceed through the block with caution.

Slipping distant (N) Slipping distants were used by the GWR to assist in the operation of slip coaches (A coach which could uncouple from an express train on the move). Slipping distant signals were attached to a distant signal and used a miniature distant arm. If the slipping distant signal was off, then the train could slip the coach in that block section. If the slipping distant signal was on however, the train had to stop to uncouple the slip coach the ordinary way. Archive photographs seem to show that no standard design was chosen on the arm, so in most cases a subsidiary or shunt signal had a notch cut into it and was repainted yellow to make the arm! One junction signal on the approach to Reading general even had a splitting slipping distant underneath to indicate which platform the train would slip at!

Backing signal (X) The backing signal was another type of signal used primarily by the GWR to help with special movements. Backing signals were placed to authorize wrong line running at stations or junctions with caution if a ground or subsidiary signal did not cover it. It became obsolete soon after the formation of British Railways. The picture shows one with a route indicator for multiple wrong line routes.

Banner repeaters (older mechanical type) (X)
Banner repeater signals show the state of the next signal when sighting is unsatisfactory. These are only used where bracketing, wrong line placement or taller posts do not help drivers to sight the signal. The more modern fiber optic version of this signal used in MAS schemes is already available in official trainsimulator content.

iii) Summary video showing the operation of Miles Platting Station Junction signalbox, now sadly gone. (Panarail productions)

Credits & sources

While I am not a signalman, I do hope that my understanding of the absolute block signalling system is correct and helpful. If you do find any erroneous details, then don’t hesitate to drop a comment below. The last thing I want to do is to spread more misconceptions. Feel free to add anything else that you feel should be in the guide in the comments and I will change it. The idea behind this guide I hope is that it is gradually shaped by the user feedback, to provide a handy and easy to read reference on a sometimes very complex system.
The signals covered are just the tip of the iceberg, as there were many different types of signals that different companies adopted over the years – especially in pre-grouping days.

With contributions from:
SacroHull (see comments.)

Sources used –
GWR 1936 general appendix: A very handy official guide to the operation of the GWR. Most regulations did not change on BR (W) what is covered in the extracts shown mostly applies to operations up to about 1970. The basics of absolute block signalling is still in force though in certain places. Extracts have been used for educational purposes should this publication not be in the public domain yet (It should be.)
signalbox.org : A very useful site covering the basic semaphore signals. There is also a nice image gallery of various signals & signalboxes.
[link] : A very realistic simulation of signalbox operations. Highly recommended if working a signalbox interests you.
Jhon C. Earwicker of Panarail productions – The youtube videos featured in this guide does a far better job of showing how signalboxes worked than I could manage.
[link] – The signalbox diagrams featured in this guide, and for information on TCB working. Another very useful site showing an overview of UK signalling as a whole.

Also, the backing & banner repeaters are available here => [link]
& here => [link]
There are also numerous other signal kits available on UKTS which provide prototypical signalling in train simulator.

But what if I want to add prototypical block signalling and make it workshop compatible?
While the majority of the Riviera in the 50s signals cannot be used in a prototypical way, there are a few official content signals which do the job. However most of these use kuju scripting which still isn’t perfect.
Official content recommended signals:
Falmouth branch (GWR wooden signals) : [link]
Cornish china clay for export (GWR steel post signals & modern colour light home as well as distant signal – [Best used in low speed situations as the signals do not clear until close/i]) :
[link]
European asset pack: The only feasible alternative for LMS / LNER UQ signals although not really based on a prototype: [link]
Isle of Wight route: Recently gained a new set of SR pattern signals. They seem to operate prototypically although should be used with caution (One such dubious case is that combined home / distant signals are used incorrectly at Smallbrook junction ) : [link]
West Highland line extension – There are a small handful of signals here which match the North British railway signals refitted with upper quadrant arms. However these signals have limited use as they are modelled to suit specific locations. :
[link]

If you want to see semaphore signalling in action on the mainlines today, then I would suggest you visit the key survivors soon such as Banbury as sadly the writing is on the wall for this type of signalling.
A very sad affair indeed is that such a handsome GWR type 7 signalbox (Banbury North signalbox) is due to be demolished by Network rail (vandalism if you ask me) . But thanks to local campaigners, arrangements have been made for visits & demonstrations inside the box before the bulldozers arrive: [link]

(Pictures taken by me)
And finally,
thank you for reading this guide.
GWRKINGCLASS.

Edit log:
V1.0 = Initial release
V1.1 (Current) = Corrected incorrect definitions on TCB/ABS.