UK railway signalling

The railway signalling systems used across the majority of the United Kingdom rail network use line-side signals to advise the driver of the status of the section of track ahead.

The current signalling is based on a two, three, and four aspect colour light system using the absolute block principle; permissive block may only be used in certain limited circumstances. 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).

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 him, a policeman would stop any following train if it arrived within (say) 5 minutes; for any between 5 and 10 minutes after, he would show a caution signal, and after 10 minutes, the line was assumed to be clear. Therefore, if a train failed midsection (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 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.

Semaphore stop signal (lower quadrant type)

Semaphore distant signal (lower quadrant type)

Combined semaphore stop and distant signals (lower quadrant type)

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 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.

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Wikipedia