The railway signalling system used across the majority of the United Kingdom rail network uses lineside signals to control the movement and speed of trains.
The modern-day system mostly uses two, three, and four aspect colour-light signals using track circuit – or axle counter – block signalling. It is a development of the original absolute block signalling that is still being used on many secondary lines. The use of lineside signals in Britain is restricted to railways with a maximum speed limit of up to . This is the maximum speed at which the train can travel safely using line-side signalling; if the train runs any faster, it will not be possible for the train driver to safely read colour-light signalling. Trains operating at speeds faster than 125 mph (for example on High Speed 1) use an in-cab signalling system that automatically determines and calculates speed restrictions.
Early days
In the days of the first British railways, "policemen" were employed by every railway company. Their jobs were many and varied, but one of their key roles was the giving of hand signals to inform engine drivers as to the state of the line ahead. They had no means of communication with their colleagues along the line, and trains were only protected by a time interval; after a train had passed them, a policeman would stop any following train if it arrived within (say) 5 minutes; for any between 5 and 10 minutes after, they would show a caution signal, and after 10 minutes, the line was assumed to be clear. Therefore, if a train failed within a section (as was very common in the early days), the policeman controlling entry to the section would not know, and could easily give a 'clear' signal to a following train when the section was not in fact clear. The number of collisions which resulted from this, as well as the introduction of the electrical telegraph, led to the gradual introduction of the absolute block principle; all systems of working other than this (including time-interval and permissive block) were outlawed on passenger lines in 1889, and all passenger lines were suitably equipped by 1895.
As train speeds increased, it became increasingly difficult for enginemen to see hand signals given by the policemen, so the railways provided various types of fixed signals to do the job, operated by the policemen, or signalmen as they soon became known (it is due to this that British railway slang still names signalmen as "Bobbies"). Many types were devised, but the most successful was the semaphore, introduced in 1841 and soon becoming widespread, although some other types did linger on until the 1890s.
Running signals
The terms "on" and "off" are used in describing British railway signals. When describing an older semaphore, "on" refers to a signal arm in the horizontal position, and "off" means a signal raised upwards or lowered downwards from pivot point (at up to 60°). With regard to newer colour-light signals, "on" is synonymous with the most restrictive aspect, while all other aspects are considered to be "off". A way to remember this is to refer to the state of the red light, or yellow light if the signal is a distant and incapable of displaying a red aspect. If it is lit, the signal is "on", and if the red light is unlit, the signal is "off".
Semaphore signals
The traditional British signal is the semaphore, comprising a mechanical arm that rises or drops 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 pull-wire but lower-quadrant signals require a heavy counterweight with push-pull rod between counterweight and arm linkage (generally assisted by the "spectacle" that carries the coloured lenses for use at night) to do that, while upper-quadrant signals fall back to "danger" under the weight of the arm.
During the 1870s, almost all the British railway companies standardised on the use of semaphore signals, which were then invariably of the lower quadrant type. From the 1920s onwards, upper quadrant semaphores almost totally supplanted lower quadrant signals in Great Britain, except on former GWR lines and their succession to BR(WR) and latterly Network Rail Western Zone.
There are two main types of semaphore; stop and distant. The stop signal consists of a red, square-ended arm, with a vertical white stripe typically 9-12 inches (230–300 mm) from the end, and advises the driver whether the line immediately ahead is clear or not. A stop signal must not be passed in the horizontal "on" (danger) position, except where specially authorised by the signaller's instruction. By night, it shows a red light when "on" and a green light when "off" (clear). The green light is usually produced through the use of a blue spectacle lens, which produces green when lit from behind by the yellowish flame from a paraffin lamp.
The other type is the distant signal, which has a yellow arm with a 'V' ("fishtail") notch cut out of the end and a black chevron typically 9-12 inches (230–300 mm) from the end. Its purpose is to advise the driver of the state of the following stop signal(s); it may be passed in the "on" position, but the driver must slow their train to be able to stop at the next stop signal. When "off", a distant signal tells the driver that all the following stop signals of the signal box (and/or until the next distant) are also "off", and when "on" tells the driver that one or more of these signals is likely to be at danger. By night, it shows a yellow light when "on" and a green light when "off". On many branch lines and short block sections, a distant signal was often fixed at 'Caution', standalone or mounted below a Stop semaphore, and so exhibited only a yellow light at night.
Where a signal consists of a combination of a stop and distant arms a mechanism is included to prevent the distant arm clearing while the stop arm is at ‘danger’.
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.
Colour light signals
In total, colour-light signals in the UK display seven aspects. These are:
Green – Clear. The train may proceed subject to any speed restrictions applying to the section of line or to the train itself. (See also Flashing Green below.) Double yellow – Preliminary caution. The next signal is displaying a single yellow aspect. Flashing double yellow - indicates that the next signal is showing flashing yellow. Single yellow – Caution. The driver must prepare to stop the train at the next signal. Flashing single yellow - warns that a diverging route is set. The next signal will be a steady single yellow with a junction indicator. Red – Danger/Stop. Additionally, on the trial section of line between Peterborough and York: Flashing Green - the train may proceed at line speed. Where this aspect is in use, the steady green aspect means that the next signal shows double yellow.
The green aspect and the four yellow aspects are known as 'proceed aspects', as they allow the train to pass the signal; the red aspect requires the train to stop.
Two-aspect systems use pairs of signals, as a follow-on from semaphores, with the rear signal in the pair using yellow and green only as a caution signal, and the signal in advance of that using red and green only as the corresponding stop signal. Three-aspect systems include yellow. Four-aspect signalling, which also includes double yellow, is mostly used on busy routes to allow shorter headways, and on fast routes to provide longer braking distances.
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. A flashing double yellow (only used in 4-aspect signalling) 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 beyond the junction is at danger (red). This sequence of increasingly restrictive aspects forces the driver to slow the train down in preparation for stopping at the red signal, and this ensures that the train crosses the junction at the appropriate speed. As the train nears the junction, the red signal beyond may 'step up' to a less restrictive aspect depending on the state of the line ahead.
The two yellows in a flashing double-yellow flash in unison rather than alternately, but the flashing double-yellow and single-yellows are not synchronised.
Flashing yellow signalling contains an additional safety vital relay typically referred to as Flashing Lamp Proving Relay (FECR) – this changes over the supply for the yellow signal transformers at each signal where flashing aspects are provided from a steady 110Va.c. to a "flashing" supply switched on and off at about 1.2 Hz or 70–72 c.p.m., once the junction points have been set, locked and detected correctly for the lower–speed divergence.
This supply has to be specially provided, either from the power–box or control centre, or by a specially designed signal control module in more modern LED installations. The increased complexity in providing flashing aspects prior to the introduction of solid state interlocking resulted in more stringent criteria for the use of flashing aspects in earlier installations.
Failures If a signal is to malfunction and not show any aspect, the driver is required to treat the blank signal as if it was red and contact the signaller.
A failure of the changeover relay to switch on the flashing indication to the double-yellow aspect would not be a problem as it is considered that a steady double-yellow followed by a flashing single-yellow aspect sequence is acceptable.
However, safety circuitry is connected to the single-yellow flashing supply to ensure that a failure of the single-yellow to change over to the flashing supply would abort the "approach release from yellow" sequence and re-impose the normal "approach control from red" sequence as failure of the single yellow to flash following a flashing double-yellow is considered potentially very dangerous.
Lens placement and alignment
The design considerations determining the familiar arrangement of roadway 'traffic signals', with red at the top, do not apply to the railway. In particular, there is no risk that a signal will be masked by a tall vehicle in front of the driver. Furthermore, to position the red aspect at the lowest position may reduce risk of obscuring of that lens by heavy snow or ice. There are standard arrangements of the lights, however unusual variations, such as horizontal mounting is allowed when demanded by local conditions or geography, such as in tunnels, areas of limited clearance, or the presence of bridges over the railway.
On pole- and gantry-mounted railway signals the most restrictive aspect is usually the lowest of the principal aspects. This places the most restrictive aspect nearest to the driver's eyeline and also reduces the possibility of the lens becoming obscured by snow building up on the lens hood of an aspect below. Similarly, on ground-mounted signals the most restrictive aspect is positioned as the highest of the principal aspects; this again places the most restrictive aspect nearest to the driver's eyeline and reduces the possibility of obscuration through snow build up.