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UK railway signalling
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UK railway signalling
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 125 miles per hour (201 km/h). 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.
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.
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".
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.
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UK railway signalling
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 125 miles per hour (201 km/h). 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.
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.
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".
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.
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