|1. Running Signals.
It could be argued that all signals are colour light signals as Semaphores display a light after darkness, in fog and during falling snow! Furthermore, there were some "mechanical colour light" signals where the spectacles of semaphore signals were used without the arms in places where it was permanently dark. Examples of this were the Sutton line platform starters at Wimbledon which were in permanent semi-darkness under a long overbridge. However, for the purposes of this article, we will consider "proper" colour light signals. Unlike semaphore signals, colour-light signals can be capable of showing a variety of indications. Signals can be single aspect (searchlight signal) where the colour of the single light changes or, as is the rule on the Southern, multi-aspect where each colour has its own light. At it's simplest there is the two aspect signal that will show either a red or green or a yellow or green aspect. These are similar to the semaphore Stop and Caution signals. There are also three and four aspect signals that will show a red, a green, one or two yellow aspects. These, obviously, combine the purpose of the Stop and Caution signals into one.
A basic red or green signal fills exactly the same purpose as its semaphore equivalent, as does the yellow and green signal. However, with a three aspect signal a red light means stop, a yellow proceed with caution and be prepared to stop at the next signal and green that the following signal is showing a yellow or green aspect. With a four aspect signal red means stop, yellow means proceed with caution as the next signal is red, double yellow means proceed with caution as the next signal is yellow whilst green means the next signal is showing a double yellow or green aspect.
Left: Three aspect colour light starting signal at Portslade, looking towards Shoreham
photograph by Glen Woods
With a full colour light installation the points and signals can be operated from lever frames but would most likely be operated from a panel. With modern installations a lot of the signals are fully automatic and are operated by the trains as they proceed along the track, activating the track circuits as they go. In a four aspect Multi Aspect Signalling (MAS) area, for example, a train passing a signal will return that signal to a red aspect, the previous signal to a single yellow, the one prior to that to a double yellow and the one before that to green. Route selection at junctions and other locations with complex trackwork may also be done automatically by computers which match the passage of trains against pre-programmed timetables.
Junction signals can be similar to semaphores, with a separate head for each line, or more commonly have a single head with one or more junction indicators. These are a series of five white lights in a straight line, at an angle from the signal head, that only illuminate when the signal is showing a proceed aspect. Many of these may be attached to one signal which is quite a saving on signal heads! The white light nearest to the signal aspects will be common to all the junction indicators fitted. Where two or more junction indicators are installed they apply to the routes ahead from left to right reading around the signal head in a clockwise direction from the 6 o'clock position. As the term junction indicator suggests, these normally apply to diverging routes with the proceed indication for the main route normally being given by the main signal head alone, without any junction indicators illuminated.
Right: The four aspect colour light starting signal at Three Bridges, with Junction Indicator, signalled for the Crawley and Horsham line. When the signal shows yellow or green aspects without the junction indicator illuminated, then the road is signalled for the Brighton line.
photograph by Nick Beck
Left: Position light signal at Three Bridges.
photograph by Nick Beck
This is a simplified definition as position light signals can be used in various situations, and old ones are still in use that display a single red and single white aspect for "stop" where the modern ones show two reds. They all have just three lights, however, with the modern ones using electronics to switch the colour of one light between red and white. There is also a version which shows two yellow lights, or a yellow and white light, which is comparable with the yellow shunt dummy with the black stripe.
The function of a position light signal is simple, when it is "off" it permits the driver to pass, but unlike a running signal, only for as far as he can see the line to be clear. It gives him permission to proceed towards the next signal (or buffer stop) only as far as the line is clear and he must be prepared to stop short of any obstruction at any time after passing the signal.
Right: This sign, at Dorking, advises drivers that they must obey a 15mph speed restriction when using the crossover from one track to the other.
photograph by Nick Beck
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This page was last updated 11 November 2003