|This is a very complex topic and this article only sets out
to "scratch the surface". Although there are sets of general rules
that apply across a whole system, these can often be amended on a local basis
leading to many exceptions that may well occur in just one particular
In the earliest days of railways, trains were not really regulated at all - they were despatched at intervals with a minimum time allowed between them. As railways grew and technology improved, both the number and speed of trains increased, leading to inevitable mishaps. To try to regulate things better, Railway Policemen were employed and they were literally a form of human signal, policing the traffic on their line and endeavouring to keep the trains a safe distance apart. (They were, naturally, nick-named "Bobbies", a name which was subsequently inherited by Signalmen and is still in use to this day). The human knack for misunderstanding and error soon led to the development of a form of mechanical signalling controlled from a central point, the signalbox.
Much improved as this system was, there were still plenty of opportunities for error, leading to many well-documented accidents, so that eventually Parliament had to intervene and formally regulate the various railway companies to make them safer. There have been many, many different Acts of Parliament covering all topics of safety but probably the most important was the Regulation of Railways Act of 1889 which made compulsory:
This meant a lot of work for some of the railways, such as the Cambrian, which had (through their dire financial conditions) been unable to make much effort to improve the safety of their lines. The LB&SCR, however, was one railway at the forefront of railway safety, having had the misfortune of some spectacular accidents early in its life and the will to try to do something to prevent recurrences!
One early signal was the "Automatic" signal invented by CF Whitworth. Far from being "automatic" in operation, this was merely a signal that was operated by the Policeman but returned to 'danger' once the train had passed, by means of a treadle. There was one of these at each end of Clayton tunnel, just north of Brighton, and it was the failure of the Policeman to see that the signal had not returned to danger that led to the worst ever accident on that railway. John Saxby worked for the LB&SCR and, in 1856, developed his "simultaneous motion" signal, installed at Bricklayers Arms Junction, that acted as a point indicator. A system of slotted link handles allowed any signal to be set at danger, even though the points were set for that route, meaning that a route could not be signalled unless the points were set accordingly. In 1860 Saxby made a locking frame for use at Victoria and in 1863 set up the business of Saxby and Farmer at Haywards Heath, where he had a virtual monopoly of the LB&SCR's signalling work for the next forty years
Large railways developed their own systems whilst smaller ones would purchase "off the shelf" from companies such as McKenzie & Holland, Saxby & Farmer or Westinghouse. As time progressed signalling systems became more and more advanced, with more and more complicated locking systems. Originally mechanical, these subsequently became electrical as well though the principle is the same with both varieties. Many different types of mechanical interlocking were designed, such as the "tappet" system which works with sliding bars attached to the signal levers that have cut-outs in them that engage with dogs on other sliding bars that move at right-angles to the first bars. According to which levers are pulled, others may or may not be pulled, thereby locking a route and its signals. Electrical locking has the same end-result but works through contacts engaging or disengaging.
But, different as their appearance might be, the basic functions are all the same. By and large you can divide signals into three categories, running signals which control trains along their route, shunting signals which control slow speed movements over short distances and information signals.
There are many, many books on the subject, some of which are little more than albums, others technical tomes. Recommended ones that are both readable and packed with useful information are:
These pages prepared in conjunction with John Divine and Chris Osment.
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Lever Frames |
1931 Bell Codes | Rule 55 | Single Line Operation | Fog Signalling | Walkers Train Describer
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This page was last updated 10 March 2005