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The UK railway signalling system used across the majority of the United Kingdom railway network uses colour lights to tell the driver the status of the section of track ahead. The most common signal type has four lights arranged as follows from top to bottom:
- Yellow
- Green
- Yellow
- Red
For a four-aspect signal as above, the main aspects are:
- Red - Stop
- Single yellow (lamp next to the red) - Caution, be prepared to stop at the next signal.
- Double yellow (both lamps) - Preliminary caution, the next signal is displaying a single yellow aspect.
- Green - Clear, you may proceed at the highest permitted speed.
The single-yellow, double-yellow and green aspects are known as 'Proceed aspects' as they allow the train to pass the signal; the red aspect always requires the train to stop.
Not all lines use four-aspect signalling - three or two aspects are used where the headway and line speed allow. The three-aspect version uses three colour lights, omitting the top yellow. The two-aspect version has only the red and green aspects, with distant signals or repeaters (signals that can show only green or yellow) giving advance warning of a red.
UK signal colour order
The green above red colour order (as opposed to the red above green used on roads) is mainly for reasons of snow. Each signal lamp has a hood to shade it against glare from the sun. If snow builds up on top, the lamp above can be obscured. Putting the most important lamp, the red, at the bottom means that there is nothing below it for snow to build up on -it will always be visible even if the others are obscured.
The requirement of road signals that a stop light should be visible over a queue of traffic and as far back as possible (thus demanding an order with red above) does not apply to railways with a block signalling system as there is no traffic queuing within a block, there being only one train in a block at a time. With railway signals, red need not be at the top for maximum visibility over distance as the driver of a train will already be expecting a red signal, having passed a yellow one in the previous block. This advance warning is missing with road signals.
The red light of a multi-lamp signalling system is also positioned so that it is nearest to the line of sight of the driver, meaning that it is at maximum brightness through its lens. For this reason running signals on the ground have the red lamp at the top.
There are many variations on this basic theme, depending on the track layout (whether there are junctions, cross-overs, stations, bay platforms, etc. and at interfaces between areas with signalling systems with different numbers of aspects.
Unusual signal aspects
There are some more unusual signal aspects in use.
- Flashing yellow - a flashing single or double yellow indicates that a train is to take a diverging route ahead with a lower line speed than the main route, indicating to the driver to slow the train down in time for the speed limit of the diverging route. A flashing double yellow means that the next signal is showing flashing single yellow. A flashing single yellow means that the next signal at the junction is showing (steady) single yellow with an indication for a diverging route, and the signal after (in advance of) the junction may be red. When the train has neared the junction and slowed down, the junction signal will 'step up' to the correct aspect depending on the state of the line ahead.
- Flashing green - flashing green aspects are on the East Coast Main Line north of Peterborough. They were installed for 140 mph (225 km/h) running in connection with the testing of the new "225" electric trains, with a steady green limiting test trains to the normal speed limit of 125 mph. Though no longer have official meaning, but they remain in place and there are a couple of locations where the presence or absence of flashing provides useful information to drivers.
- Splitting distants - at some locations approaching a junction two heads are placed side by side. When this signal or the junction signal is at danger, one head is dark and the other shows red or single yellow. When the junction signal is not at danger, both heads show an aspect: the one for the route set ahead of the junction (left or right) shows the correct aspect while the other shows single yellow (or double yellow at an "outer splitting distant").
- Green over yellow, or green over green - the London Underground uses separate red/green "stop" and yellow/green "repeater" signals. If a repeater signal is at the same location as a stop signal, it is placed underneath it and lit only when the stop signal is green. Thus the order of the heads is (from top to bottom) green, red, green, yellow, and aspects are red, green over yellow, and green over green.
- Yellow over green - this was used in the experimental "speed signalling" at Mirfield, abolished in 1970, to provide an additional caution. It meant that the next signal was showing double yellow.
Proceed on Sight Authority
This new concept will have an additional aspect to allow the signalman to authorise drivers to pass signals when they are at red due to influences within the interlocking. The term may be abbreviated to "PoSA".Railway Group Standard GE/RT8071Semaphore signals
See also railway signal
The traditional arrangement in Britain was to use a mechanical signal that raises or lowers to indicate 'clear' (an "upper-quadrant" or "lower-quadrant" signal). Both types are fail safe in the case of a mechanical problem, but lower-quadrant signals require a heavy counter-weight (usually in the form of the "spectacle" that carries the coloured filters for use at night) to do that, while upper-quadrant signals return to danger under the weight of the arm.
Other indicators
Shunting signals are much smaller and are laid out in a triangular formation. Two red lights or a red and a white light side by side mean you may not proceed. The proceed aspect is two white lights placed diagonally, the red light(s) having been extinguished, and instructs a driver that they may proceed but be prepared to stop short of any obstruction. If a shunting signal shows one yellow and one white light or two yellows, you may pass the signal along the route for which it cannot be cleared (for example, a shunting neck).
Subsidiary aspects appear below or beside running signals. They consist of the two white lights placed diagonally; when lit, with the main aspect showing red, they instruct the driver to proceed but be prepared to stop short of an obstruction (for example, to enter an already-occupied platform). When unlit the driver obeys the main signal aspect.
Junction indicators (colloquially known as "feathers" or "lunar lights", or "horns" in Scotland) are used to indicate which route is set at a junction. They consist of a row or diagonal of five white lights (previously three on the Southern Region). Where more than one diverging route is possible, a selection of rows and / or diagonals can be used. When the primary route is set, the feather is not illuminated (unless all routes are of a similar speed, in which case there is a feather for each route). When a diverging route is set and any approach release conditions are satisfied, the respective feather is illuminated.
Route indicators are alphanumeric displays above or next to a signal to display either the line or platform to which the train is been routed. In the past they were typically white lamps illuminated in a dot-matrix fashion to give an alphanumeric display, but new installations use fibre-optic displays driven from a single lamp.
Notes and References
See also
"UK railway signalling"