For some insane reason over on the Model Rail Cast forums I offered to create a piece on UK signalling after someone offered the same for US. So here goes my attempt at covering the basics of UK signalling from early days to the current day. This is far from complete (see sources below for more information) and may contain errors (see comments below to see if any have been found).
There is much I have not even touched on in this piece such as the cab equipment, shunting, interlocking or the unique signals used before BR days - maybe I'll do a part 2 soon.
Back in the 1830s when railways were just getting started here there was no signalling, or train control, of any type. Trains were driven like cars - by sight, if the driver saw something ahead he had to stop otherwise he was free to go.
After some notable accidents the companies started to ensure that trains were separated by time (they were given a 10 minute headway). Policemen would stand at the side of the line with three flags. The red one was shown in the first half of the time (5 minutes) and the train had to stop. The yellow one was shown as a caution to the driver in the last half of the time and the green was shown only after the full time had elapsed. Although slightly safer it was still not ideal a number of collisions resulted from trains breaking down or as the result of a fast train following a slow one. However companies are only interested in profit and as the line capacity needed to be improved the headways reduced and the numbers of accidents increased.
In response to the number of accidents under the early systems of train control it was decided to separate trains not by time but by space. The lines were broken down into blocks, with only one train being allowed in each block (an idea which has survived over 150 years). Mechanical signals were placed at the entry to each block and would show either a stop aspect if the block ahead was occupied or a proceed aspect if the block was vacant.
A semaphore signal displays stop by lowering the arm and placing a red spectacle in front of the lamp (which is lit at night or in bad weather). Proceed is shown by raising the arm and placing a blue spectacle in front of the lamp, blue because the oil lamp burns yellow and the combination result in a green light. Another time of signal the distant (as opposed to the home signal shown above) is yellow and will use either a yellow or green light. The ditant signal is simply a repeat of the following home signal and gives prior earning of a atop signal ahead.
They are controlled from signal boxes which are connected by telegraph, allowing the signalmen to communicate and check if a train can be sent on to the next station. Further additions to this basic technology allowed the addition of interlocking (disallowing the signalman from allowing conflicting/dangerous moves) and track circuits (which show the presence of a train on track by lighting a light).
Where a train had a choice of different routes (say on the approach to a main station) one signal would be provided per route, this is what is behind the many photos of impressive looking arrays of signals at places like Edinburgh or Kings Cross. Several semaphore signals remain in place today, the photo is of one at Arbroath station on the mainline between Dundee and Aberdeen.
As track circuits were fitted to more and more of the track miles it was realised by managers that signalmen no longer needed to see the track they were controlling. This lead to the advent of PSBs (power signal boxes) and later to IECCs (Integrated Electrical Control Centres) where the points and signals were controlled remotely, allowing one person to control more of the railway. This also allowed for the signalling to be replaced with signals giving more than just a stop/proceed message.
There are two main types of colour light signals 3 aspect and 4 aspect, the only difference being the number of different signals which can be displayed. A three aspect signal shows red for stop and green for proceed but adds in yellow which means "caution (proceed but expect the next signal to be red)". 4 aspect signals add in a forth message, double yellow "advance caution" meaning expect the next signal to be showing yellow.
The chance was also taken to simplify the method of telling the driver which way his train would go, two methods are used. Generally rows of white lights above the signal but to either the right or left are lit when a diverging route is set. When space does not allow for this or where there are too many possibilities a theatre indicator is used. This is a set of small lights which display either a number or a letter.
The top photo is of a 3 aspect signal at Dundee (on the East Coast mainline), the theatre indicator is showing W (either for Waverley (to the south) or for West (to Glasgow). The bottom photo being of the two 4 aspect signals at the south side of platforms 3&4 at Haywards heath on the London - Brighton mainline.
Single Line Working
Occasionally it is necessary to lay a single line rather than the more common 2 or 4 track mainlines, an example still in use today is the West Highland Line.
The simplest method of ensuring no collisions on a single line is the "one engine in steam" rule, simply only one train with the ability to move (ie steamed up) is allowed onto the entire length of single track at a time.
This was soon replaced by a token system, where each length of single track (or part length of single track) had a unique token and only the drives in possession of this token was allowed onto the length of track it protected. Several trains travelling in the same direction could be allowed by showing the token to each driver and passing it to the last one - hence avoiding the problem of the token being at the wrong end of the track.
This workaround evolved into electronic token block working whereby a machine with several tokens was provided at each end, when a train exited the section the token was returned. The machines where electronically interlocked so that only 1 token could be removed at a time.
And this later on evolved into Radio Electronic Token Block where the tokens where transmitted by computers connected to radios in both a central control room and the train cabs. This system is still in use on a number of lines and is due to be replaced with the European Rail Traffic Management System system by 2011.
Sources / Further Reading