Passenger railways developed very quickly in the 1830s but with infrequent trains, slow speeds and light stock there was not much need for signalling in the early days. When services became more frequent and faster some regulation became necessary and railway policemen were given the job of controlling the trains as well as the passengers. They were later called 'signalmen' but because of the connection, are still nicknamed 'Bobbies'.
Crude signals of many types that could be seen by train drivers a long distance away were brought in, together with auxiliary or distant signals that allowed for braking distance; red, green and white flags and lights also became commonly used. Rule books, detonators, semaphore signals and the Railway Inspectorate all came in the 1840s. The usual method of train working at that time, and on some railways for nearly fifty years, was Time Internal. In this, an interval of about five minutes had to elapse before a train was allowed to follow a previous one and a green flag or light was shown. When an interval of about ten minutes had elapsed a white flag or light was exhibited. This system did not give much protection if a train came to a stop in a section and the Guard could not get back far with detonators. The electric telegraph was used early in the 1830s for normal railway working but apart from protecting traffic through a few tunnels was not much used for signalling. The South Eastern was a pioneer in the use of the telegraph for signalling; and in 1851, they installed a very simple form of Absolute Block working using single stroke bells'. The signal codes were recorded in what is still called a Train Register and the system improved safety to a considerable extent. Few other railways seem to have copied it though.
At this time Greenwich Time was sent around the country by the railway telegraph systems from the Greenwich Observatory. Until the 1850s most points and signals were worked independently with no interlocking but by the late 1850s crude types of interlocking, with everything controlled from one frame, were introduced at a few busy junctions and early signal boxes with signals mounted above them started to appear.
Interlocking was very expensive and a lot of railways did not install it for many years until forced to by legislation Absolute Block at last started to become popular using instruments that indicated the state of line. Many types of instruments were used and some of them were in use for a hundred years or more. In 1874 Sykes introduced their Lock and Block instruments which were mechanically (later electrically) connected with the signals to ensure that the correct sequence of operation had to be carried out. Treadles to prove where a train was were also soon introduced; not many railways though used Lock and Block because of its cost.
Single line working was a big problem when intensive traffic came about and working by pilotman, staff, staff and ticket, one engine in steam, etc, sometimes in conjunction with block instruments, were used but the big break through came in 1878 when Tyers introduced their electric tablet system that was completely flexible in operation. This was followed by the Webb and Thompson, key token and other types most of which were in use for many years.
The relationship between braking distances and signalling was very important and a lot of experiments were carried out in the 1870s with several different types of continuous brakes. It was found that a light train fitted with the Westinghouse air brake could stop in 440 yards from 60 mph and this distance is, I think, still the standard overlap used with mechanical signaling. Track circuits, which electrically detect where a train is, were experimented with in America and, without much success, in Britain, by Sykes. The big problem in Britain was the widespread use of the wood centred Mansell Wheels, which could not act as a short circuit. In later years earthing straps were fitted to the wheel sets to overcome this trouble. In 1886 a large installation of track circuits was put in at St Paul's with some success but they were not widely introduced until the Edwardian period after a series of terrible accidents, which their use would have prevented. Again in America automatic signalling was tried out, including the Halls clockwork signals. In 1881 there was a terrible accident near Armagh caused by the lack of a continuous automatic brake and the use of Time Interval working on a single line with heavy gradients. At last legislation was brought in and proper brakes and working practices were made compulsory on all railways from then on. Signal boxes had become very large by the turn of the century and the mechanical working of points over large distances was very difficult. In America 'power' boxes were being introduced and the GER installed an air worked installation at Granary in 1899 with great success. The L&SWR brought quite a long air worked automatic stretch into use in 1902 again using American equipment and this worked very well for some sixty years. I believe that one of the power frames is still in use at Salisbury. The L&NWR developed a successful all electric system at this time.
This again, although rather basic by modern standards, stayed in use a very long time.
Colour light signals and three position electrically operated signals started coming in to use about the early 1920s but the big problem in Britain was what colour aspects to use. The majority of railways still used red glasses in their distant signals and this could not be used in the same way with colour light signals. In 1924 a Signal Engineers Conference decided to advocate the use of upper quadrant signals with yellow aspects instead of red. At the same time distant signals started to be painted yellow as well. In the 1920s very large power boxes were built and four aspect colour light signals were first introduced on the Southern. One of the largest power boxes was London Bridge, built in 1928 with 311 levers. Because of its great length the mechanical interlocking of the miniature levers caused a lot of problems. In 1929 North Kent box was the first built with all electric interlocking which eliminated the problem and was much cheaper to install and alter. Even power boxes took up a lot of space and experiments were carried out in the 1930s with route relay systems using panels and switches instead of miniature levers to make them much more compact. Many different arrangements were used but after the War the NX system combined with miniature relays became very popular. Nowadays Solid State interlocking is often used and radio signalling for single lines. Cab signalling is used in some countries but not as far as 1 know in Britain. The Docklands Light Railway uses computer control and the Victoria Line has automatic control, the operator being able to take control in an emergency.
this first appeared in GIHS Newsletter January 2005