UK railway signalling

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.

ignals

"See also: Railway signal" and "British railway signals".

emaphore signals

The traditional British signal is 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.

During the 1870s, all the British railway companies standardised on the use of semaphore signals, which were then invariably all of the lower quadrant type. [Vanns, M.A., (1997), "An Illustrated History of Signalling", Ian Allan, ISBN 0-7110-2551-7, p.25 ] From the 1920s onwards, upper quadrant semaphores almost totally supplanted lower quadrant signals in Great Britain, except on former GWR lines. [Vanns, M.A., (1995), "Signalling in the Age of Steam", Ian Allan, ISBN 0-7110-2350-6, p.80 ]

Current British practice mandates that semaphore signals, both upper and lower quadrant types, are inclined at an angle of 45 degrees from horizontal to display an "off" indication. [ [http://www.rgsonline.co.uk/docushare/dsweb/Get/Rail-41399/Rt0031a.pdf Railway Group Standard GK/RT0031] ]

Colour light signal aspects

The main aspects are:

* Red - "Danger/Stop"
* Single yellow - "Caution", be prepared to stop at the next signal.
* Double yellow - "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 not equipped with a red aspect) giving advance warning of a red aspect.

Flashing yellow aspects

A flashing single or double yellow aspect 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 (only used in 4-aspect territory) 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 may 'step up' to the correct aspect depending on the state of the line ahead.

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, crossovers, 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 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 InterCity 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 Liverpool Loop Line and London Underground use 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.

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 are associated with main running signals, and are mounted below or beside the main signal. When used at colour light signals, they consist of two white lights at 45 degrees, normally unlit. 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, 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 line of five white lights (previously three on the Southern Region). Where more than one diverging route is possible, a selection of lines at different angles can be used. When the highest-speed 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 'straight' route, and where a maximum of three routes are to one side of the 'straight' 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. At certain locations, no route indication is given for the highest-speed route.

Preliminary Routeing Indicators (PRIs) are installed on the approach to certain junctions. When the junction signal is displaying a 'proceed' aspect, the PRI will display an arrow. The arrow points up when the highest speed route is set. When a diverging route is set, the arrow points in the appropriate direction (mimicking the junction indicator on the junction signal). This advance indication gives the driver an opportunity to stop before the junction points, if wrongly routed. At the present time, PRIs are few in number, but they are likely to become more common.

Proceed on Sight Authority is a new concept which introduces 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". [ [http://www.rgsonline.co.uk/docushare/dsweb/Get/Rail-41368/GERT8071.pdf Railway Group Standard GE/RT8071] ]

SPAD indicator is a separate indicator which may be positioned after a main signal where there is a likelihood of a serious collision at a junction if a SPAD (signal passed at danger) occurs at the main signal. SPAD indicators are mounted against a blue backplate or surround. They are normally unlit but following a SPAD they display a steady red light between two flashing red lights arranged vertically. Drivers must stop their train immediately when this indicator is illuminated.

Obsolete signals

Purple lights were used on some early signals in particular circumstances (e.g. wrong-road or goods lines). [ [http://www.trainweb.org/railwest/railco/sdjr/x-arms.html S&DJR wrong-road signals] ]

Three-position semaphore signals

From 1914, a small number of British installations used motor-operated three-position semaphore signals of North American origin. These worked in the upper quadrant to distinguish them from the two-position lower quadrant semaphores that were standard at the time of their introduction. When the arm was inclined upwards at 45 degrees, the meaning was "caution" and the arm in the vertical position meant "clear". Thus, three indications could be conveyed with just one arm and without the need for a distant arm on the same post.

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. 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 occurred 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.

Warning systems

Because of the propensity for heavy fog in some parts of the British Isles, fog signal rules were established on the UK railway system to keep train traffic moving without incurring the severe delays that would be necessary if drivers had to stop or travel slowly up to each signal and read its indication. During heavy fog, fogsignalmen would be stationed at distant signals with a lantern and detonators — small explosive charges that could be strapped to the rail to be exploded by the wheels of a train. The fogsignalman's duty was to repeat the indication of the signal using his lantern; the semaphore arm was usually obscured by fog and hence invisible to the driver of a moving train. If the distant signal were displaying 'caution' (warning that a signal ahead was at 'danger'), the detonators remained on the rail and the fogsignalman would show a yellow lamp to show 'caution'; if the distant signal were clear, the detonators would be removed from the rails and a green lamp would be displayed.

Britain's Great Western Railway introduced the Automatic Train Control (ATC) system in 1906. This system is the forerunner of today's Automatic Warning System (AWS) and consists of an electrical system that sounded a bell in the cab as the train approached a signal at clear. Power was fed through a metal ramp to a pickup on the underside of the locomotive to power the bell. An absence of the electrical voltage on the ramp caused a warning horn to sound in the locomotive's cab. The driver then had a set time to acknowledge the warning and start braking his train accordingly. If the driver did not acknowledge the warning, the brakes would be applied automatically. Where this was implemented, it did away with the need for fog signalling, since the driver could tell the state of the distant signal regardless of his ability to see it.

The current system of AWS in use on Britain's railways is similar in principle to the Great Western's ATC but does not rely on physical contact between the track equipment and the train; instead an inductive system is used.

On passenger lines, AWS is now supplemented by the Train Protection & Warning System (TPWS). TPWS will automatically apply the train's brakes in the event of a fitted signal being passed at danger.

Notes and references

External links

* [http://www.rgsonline.co.uk/docushare/dsweb/Get/Rail-41399/Rt0031a.pdf Railway Group Standard: GK/RT0031 Lineside Signals and Indicators]
* [http://www.signalbox.org/ Information on traditional British signalling]
* [http://www.xs4all.nl/~dodger/tech.htm Docklands Light Railway signalling system]
* [http://www.derby-signalling.org.uk/ History of Railway Signalling in the Derby area]

See also

* British railway signals
* Rule 55
* Train Protection & Warning System (TPWS)
* Automatic Warning System
* Automatic Train Protection
* Pass of Brander stone signals