British railway signals

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Modern British signalling is based on a two, three, and four aspect colour light system using non-permissive block rules. It is a basic progression of the original semaphore signalling that can still be found on many secondary lines. The use of lineside signals in Britain is restricted to railways with a maximum permissible speed of up to 125 mph (201 km/h).

Main signals

Semaphore signal indications

  • Semaphore stop signal (lower quadrant type)

  • Semaphore distant signal (lower quadrant type)

  • Combined semaphore stop and distant signals (lower quadrant type)

Semaphore signals in Britain exist in both lower quadrant and upper quadrant forms. Both types have the same meanings. From the 1920s onwards, upper quadrant semaphores almost totally supplanted lower quadrant signals in Great Britain, except on former GWR lines.[1] Current British practice mandates that semaphore signals, both upper and lower quadrant types, are inclined at an angle of 45° (+20°/-10°) from horizontal to display an off indication.[2]

A stop signal comprises a red arm with a square end and a white band. In the horizontal position, the meaning is stop (red light at night). When the arm is inclined, the meaning is proceed (green light at night). A distant signal comprises a yellow arm with a fishtailed end and a black chevron. In the horizontal position, the meaning is caution (yellow light at night). When the arm is inclined, the meaning is proceed (green light at night). When stop and distant arms are mounted together on one post, the stop arm is positioned above the distant arm, and the two are interlocked so that the distant signal will not come off unless the stop signal above it is also off. This is done to prevent confusion in cases where a pair of stop signals is mounted one above the other, and in such a case red over green would mean clear for right hand divergence (this is not common practice). A distant signal in the off position proves to the driver that the following stop signals controlled by the same signal box are also off, and therefore means that he does not need to slow his train.

Colour light signal aspects

  • Green

  • Double yellow

  • Yellow

  • Red

Clear (green)
The track ahead is clear. The train is permitted to travel at any speed up to the current line limit.
If the signal is a 4-aspect signal, then the following signal will be showing clear (green) or preliminary caution (double yellow). If the signal is a 3-aspect signal, then the following signal will be showing clear (green), or caution (yellow).
Preliminary caution (double yellow)
Be prepared to find the following signal showing caution (yellow).
Caution (yellow)
The following signal will be showing Stop (red), and the driver must be prepared to stop at the following signal if it does not change to a less restrictive aspect before they reach it.
Stop (red)
Also known as danger. The driver must stop at the signal. The train may only proceed when the signal changes to a less restrictive aspect.
The red stop signal is non-permissive and must not be passed unless the driver has verbal authority from the signaller or hand signaller (the hand signaller must contact the signaller and also show a coloured flag to the driver and guard of the train passing the signal at danger.) This signal may denote that the block ahead is occupied and must not be entered for safety reasons. Additionally, a red signal will be shown on the approach to a set of points when a clear path through them is not set.

Junction signals

  • Left route

  • Second left route

  • Route indicator

British signalling is unusual in that when a train approaches a junction and is taking a diverging route, the preceding signal will indicate the diverging route to the driver. A fixed lineside sign shows the speed that applies through the divergence. Most signalling systems around the world adopt the speed signalling philosophy, where the safe route speed is shown with no indication of the divergence at a junction.

Semaphore signals at junctions normally have a separate arm for each route, usually reading left-to-right, although in confined or hard-to-see locations the arms may be mounted above one another, reading left to right from the top downwards. Such arrangements are known as splitting signals. It is normal today to only find splitting stop signals, but traditionally splitting distants would be employed too; a series of side-by-side distant signals to give the driver advance warning of the route to be taken,so he could reduce his train's speed if necessary. Today, a single distant signal is generally used which displays caution if speed reduction is necessary. The height of each individual post of splitting signals (known as dolls) generally gives some information about each route; in some cases, the tallest doll applies to the highest-speed route; in others, the tallest doll applies to the most important route, be it slow or fast.

It is unusual to find a splitting signal with more than four dolls; if more route indications are required, a single stop arm with a route indicator - mechanical or electric - mounted underneath, is used. The route indicator may show an abbreviated version of the train's destination (usually at station exits), or a number (usually at station entrances to denote the platform into which the train is routed).

Colour-light signals which display whether the route is diverging from the route ahead will have a Route Indicator — colloquially called a feather— of white lights (originally a single, long, u-shaped fluorescent tube in an open-fronted case) attached to the main signal.[3][4] If the feather is lit, then the train will be diverging from the main route at the next junction. A feather may point to the left or to the right, as appropriate. If individual light bulbs are used, each feather usually has five, although a few older examples installed by the Southern Railway only have three. A signal will have more than one feather where there is more than one diverging route ahead. The lit feather indicates which diverging line the train will be directed to. In the example shown, there are two routes diverging to the left of the main line, and the train will take the first one of the two.

In areas where speeds are lower and there are a number of routes which can be taken, alphanumeric (also called theatre-style) route indicators are used to display a number or a letter (e.g. a platform number or line designation) to denote the route the train is to take. They may be located above or beside the relevant signal. When a route is set and the signal is cleared, the relevant letter or number is shown. On shunting signals, where speeds are much lower, a miniature version of the alphanumeric route indicator is used.

When a route is set at a junction that involves the train taking a diverging route that must be passed at less than the mainline speed, a system known as approach release is used. There are a number of different types of approach release that are used on British railways but the most often used is approach release from red. This system has the signal before the diverging junction held at red until the train approaches it, whereupon it changes to a less restrictive aspect with the appropriate direction feather of five white lights. This is required so that the signals approaching show the correct caution aspects, slowing the train down for the junction. While the junction signal is held at red, the preceding signal will be displaying caution (yellow), and the one before that will display preliminary caution (double yellow) if it is a 4-aspect signal. This system allows for a gradual decrease in speed until a safe speed is reached for the train to move through the junction.

Another common system is approach release from yellow with flashing aspects in rear. It is essentially similar to approach release from red, except that the junction signal is released from yellow and the signals in rear will flash to warn the driver that the train will be taking a diverging route ahead. Where the turnout speed is the same as the mainline speed, approach release is not necessary.

On modern high speed routes, such as the East and West Coast Main Lines, some turnouts at major junctions are designed to operate at maximum or near maximum linespeeds to keep the average speed of the journey as high as possible and reduce journey times as well as unnecessary wear on the train wheels, brakes and the track. Movable frog switches are occasionally employed to allow high speed running through the junction.

Banner repeater signals

  • Banner on

  • Banner off

  • Green banner

Banner repeater signals repeat the indication of the following signal, where the driver's view of it may be restricted, for example because of track curvature or a bridge abutment.

Banner on
The signal being repeated is showing a Stop aspect, and the train must be prepared to stop at that signal.
Banner off
The signal being repeated is showing a proceed aspect, so can be passed.
Green banner
Although not in widespread use, some banner repeaters can display a third (green) indication, meaning that the signal being repeated is showing a Clear aspect.

Shunting signals

Shunting signals allow a train to move forward onto a line which may already be occupied.

Position light signals

  • Sub signal

  • Ground position light

  • Yellow ground position light

Shunting signals may be attached to a main signal in which case they are only cleared when the shunting movement is required (known as a subsidiary signal), and they display two white lights at an angle of 45°. The driver may pass the signal with caution at a speed which allows the train to stop short of any obstruction.

These signals may also be placed on the ground called a ground position light (GPL), or on a post with no corresponding main signal. In this case, the signals will show either two red lights or a red and white light in a horizontal arrangement, when no movement is signalled.

If two red lights or one red and one white are shown, the signal must not be passed.

When a shunting movement is signalled, the signal will show two white lights in a diagonal arrangement. This means the driver may pass the signal with caution at a speed which allows the train to stop at any obstruction.

This type of signal is also used to designate a limit of shunt, the point up to which trains that are shunting are allowed to proceed. In this case, two red lights are displayed side by side, but no other aspect can be shown. No train is allowed to pass this fixed signal in the direction shown (this will be against the normal direction of travel on the track in question).

Here the two yellow lights or one yellow one white indicate a shunting movement is permitted past the signal but only for a move in a direction for which the signal cannot be cleared (for example, towards a headshunt rather than on to the main line). Again, two white lights at a 45° angle indicate shunting is permitted.

Miscellaneous indicators

Off indicator
Off Indicator
An illuminated off indication means the associated signal is showing a proceed aspect. These are mainly used at stations, for the benefit of the train-crew and platform staff. When the display is blank, it means that the associated signal is at danger.

An illuminated indication CD (close doors) is an instruction to close the train's power-operated doors.

An illuminated indication RA or R (right away) means that station duties are complete and the train may depart.

Speed restriction signs

Speed restrictions are imposed on a route to ensure a train is always travelling at a safe speed for the prevailing conditions along the track.

Permanent speed restrictions

Permanent speed restrictions are imposed where the route encounters a hazard such as a tight radius curve, level crossings, certain junctions, tunnels and bridges and where the train is entering a terminal station.

PSR warning
Warning boards

Warning of a permanent speed restriction (PSR) ahead of 75 mph (121 km/h). Typically placed about a half mile to a mile before the start of the permanent speed restriction, depending upon the difference between the maximum permitted line speed and the restricted line speed.

PSR warning diverging

Warning of a permanent speed restriction on the left diverging route of 75 mph (121 km/h). Again, typically placed about a half mile to a mile before the permanent speed restriction on the diverging route, depending upon the difference between the maximum permitted line speed and the restricted line speed.

PSR
Permanent speed restriction signs

Start of permanent speed restriction of 125 mph (201 km/h) on the main route

Differential PSR

Start of differential permanent speed restriction, with two varying restrictions for different types of trains. The figure above the line is the maximum permitted speed for freight trains, while the figure under the line is the maximum line-speed for passenger trains. In this example, freight is permitted to 35 mph (56 km/h), while passenger trains are permitted to 70 mph (110 km/h).

PSR diverging

Start of permanent speed restriction of 40 mph (64 km/h) on the diverging route to the left. Placed before the diverging route to instruct or remind the driver of the maximum line speed on the diverging route.

Temporary speed restrictions

Due to engineering works, a temporary speed reduction (TSR) may be enforced at a particular location.

Temporary speed restriction warning

Warning of temporary speed restriction of 40 mph (64 km/h) ahead.

Temporary speed restriction

The start and termination indicators of a temporary speed restriction of 40 mph (64 km/h).

A temporary speed restriction warning board showing R is positioned beyond the end of platforms where trains may stop between the warning board and the start of the restriction, to remind drivers of the restriction ahead. A warning or commencement board showing a diagonal line means that a TSR shown in the operating notices is not in place and trains may proceed at normal line speed.

Unusual signal aspects

Flashing green

During testing of the Class 91 Electra electric locomotives following the electrification of the East Coast Main Line in the late 1980s, British Rail introduced a flashing green aspect as authority to exceed 125 mph (200 km/h)—up to a maximum of 140 mph (225 km/h)—on the stretch between Stoke Junction, south of Grantham, and Werrington Junction, north of Peterborough. These aspects do not apply to trains running in normal service.

Meaning: The track ahead is clear and available for running in excess of 125 mph (200 km/h). The train is permitted to travel at a speed in excess of 125 mph (200 km/h), up to the current line limit (which may be as high as 140 mph (225 km/h)). The following signal will be showing clear (green or flashing green). Although the signals show a flashing green aspect, drivers are still not allowed to exceed 125 mph (200 km/h), and thus this system is not in use.

See also

References

  1. ↑ Vanns, M.A., (1995), Signalling in the Age of Steam, Ian Allan, ISBN 0-7110-2350-6, p.80
  2. ↑ Railway Group Standard GK/RT0045
  3. ↑ Green, Jonathon (1987). Dictionary of jargon. London: Routledge & Kegan Paul. p. 212. ISBN 0-7100-9919-3. 
  4. ↑ Vanns, Michael (1997). An illustrated History of signalling. Shepperton, England: Ian Allen. p. 58. ISBN 0-7110-2551-7. 
  • EuropeanBahn signalling guide to the East Coast Main Line Express in Microsoft Train Simulator.

External links

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