UK railway signalling
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The railway signalling systems used across the majority of the United Kingdom rail network use lineside signals to advise the driver of the status of the section of track ahead. Most lineside signals are in colour light form, but a significant number of semaphore signals remain on secondary lines.
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[edit] Colour light signal aspects
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.
The most common type of four-aspect signal has four lenses arranged from top to bottom as follows:
- Yellow
- Green
- Yellow
- Red
Modern LED signals with just two lenses can show all four aspects. The lower lens is capable of displaying a red, yellow or green light.
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.
[edit] Lens colour order
The red light of a multi-lamp signal is 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. Signal alignment is generally aimed towards a distance 200 metres in rear of the signal and at a height of 2.5-3m from the left hand rail. Ground mounted signals are rarely so critical for alignment (hence ground mounting) and are often used in tunnels where the relative luminosity of the aspects are much higher.
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.
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.
[edit] 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 employed 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. They no longer have official meaning, but 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.
[edit] 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 signal will notionally be used where the route setting and locking function is still proved to be operable but a function such as train detection or lamp proving of a signal ahead may be failed. The authority will allow the Driver to pass the signal and proceed at a speed slow enough that they may stop short of any obstruction (in common with other degraded modes of operation) The term may be abbreviated to "PoSA".[1]
[edit] Semaphore signals
See also railway signal
The traditional British signal was the semaphore, comprising a mechanical arm that raises or lowers to indicate 'clear' (termed an "upper-quadrant" or "lower-quadrant" signal, respectively). Both types are fail safe in the event of breakage of the operating wire, but lower-quadrant signals require a heavy counter-weight (usually in the form of the "spectacle" that carries the coloured lenses for use at night) to do that, while upper-quadrant signals return to danger under the weight of the arm.
[edit] Other indicators
Position light shunting signals are small signals, usually ground mounted, comprising three lenses in a triangular formation. Two red lights or a red and a white light side by side means 'stop'. The proceed aspect is two white lights at an angle of 45 degrees, the red light(s) having been extinguished, and instructs a driver that they may proceed but must be prepared to stop short of any obstruction. A shunting signal that shows one yellow and one white light or two yellows horizontally may be passed along the route for which it cannot be cleared (for example, a shunting neck).
Subsidiary signals appear below or beside running signals. They consist of two white lights at 45 degrees; when lit, with the main aspect showing red, they instruct the driver to proceed but be prepared to stop short of any obstruction (for example, to enter an already-occupied platform). When unlit, their normal aspect, the driver obeys the main signal aspect.
Junction indicators (colloquially known as "feathers", 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. These can be used where there is a maximum of six routes as well as the main route, and where a maximum of three routes are to one side of the main route.
Alphanumeric Route indicators above or next to a signal display either the line or platform to which the train is being routed. In the past they were typically white lamps illuminated in a dot-matrix fashion to give an alphanumeric display (known as "multi-lamp" or "theatre" type), but new installations use fibre-optic displays driven from a single lamp. These are usually restricted to areas where all routes are at low speed, usually on the approach or departure from large stations, but are not restricted to signalling a small number of routes like Junction Indicators. In rare cases where too many routes are provided for Junction Indicators, but the main route is at high speed, such as at a large station with high speed through lines (such as Crewe station), 2 Alphanumeric Route indicators are provided side by side and are used to indicate routes to their side of the main route, and like with Junction Indicators, none are lit when the main route is set.
[edit] Obsolete Signals
[edit] Purple Lights
Purple lights were used on some early signals in particular circumstances (e.g. wrong-road or goods lines).[2]
[edit] Euston to Watford Experimental System
This scheme, on the face of it, was a fairly standard colour light system, with each stop signal (which could show red or green) having an attendant repeater signal (showing red, yellow or green - the red used only for when the two stop signals on either side were also red). What made the scheme unusual was the provision of an automatic 'calling on' facility. The stop signals had an additional signal head ('marker light') that featured a red aspect plus a miniature yellow aspect. This marker light was mounted part way up the post. On repeater signals, the marker light was offset to the left-hand side of the post to indicate that the 'stop and proceed' rule applied. Junction stop signals were provided with two main signal heads, one mounted higher than the other. Splitting distant signals had three main heads, the centre one mounted higher than the other two.
When a train stopped at a red stop signal, its presence on the track started a time delay relay. At the conclusion of the time delay, the red marker light was extinguished and replaced by the miniature yellow (the upper red aspect remained lit). The train stop also lowered, but this was not used by the main line trains but was provided because the line was also used by Bakerloo Line trains of the London Underground. The calling on aspect authorised the driver to proceed, but to be prepared to stop short of another train.
The scheme was not considered a great success. In fact during periods of severe service disruption, it was not unusual to see several trains buffer-to-buffer along the line, though this occured when the line was much busier than now. Concern was expressed that similar coloured aspects had different interpretations depending on where on the signal they appeared.
The system was finally identified for replacement following an accident at Kensal Green, when a main line train ran into the back of Bakerloo Line train. The driver had apparently mistaken the calling on aspect for a normal yellow aspect (the signal was temporarily operating on a maximum yellow due to track side work). The indications were that the driver was distracted as his pay slip and its envelope were recovered from the wreckage - but this was never proved as the cause. It was also suggested that the driver, may not even have checked the indications having observed the fall of the train stop. The entire line was resignalled to the standard colour light system in 1988.
[edit] Notes and References
[edit] See also
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