Railway 200: Signalling

As part of the 200-year anniversary of the opening of the Stockton & Darlington on 27 September 1825, this article looks at the early developments in signalling. At first there was generally no need as trains were slow and infrequent. But five years after the Stockton and Darlington Railway opened, the Liverpool & Manchester Railway ran its first train on 15 September in 1830, believed to be the first to use a true signalling system with the ‘time interval’ system to keep trains a distance apart.

From the mid-1830s, most railways in Britain were using the time interval system. ‘Policemen’ were stationed at intervals along the track and gave a hand signal to the driver of each passing train to indicate the state of the line ahead. If a second train approached immediately the previous one had passed, a ‘danger’ (stop) signal was shown to the second driver. After a specified period of time, typically five minutes, a ‘caution’ signal would be shown to a following train. After a further time interval, a ‘clear’ hand signal was given to any following train.

Bobby

The ‘policeman’ name for the operator of the signals is where the term ‘bobby’ originated from, and this remains in use to this day in the UK when referring to a signaller. A lantern was provided for nighttime signalling with plain and coloured lenses. Most railway companies then provided similarly coloured flags for daytime use, in preference to hand signals. Fixed signals were subsequently developed and again the Liverpool & Manchester Railway is believed to have led the way, with a signal first used in 1834. A lamp for night use was provided, showing a red light for ‘danger’ or a white light for ‘clear’.

The railway companies introduced many variations of signals. These included revolving boards and balls raised and lowered on a post. The Great Western Railway (GWR) company went on to introduce a ‘disc and crossbar’ signal, superseding its red ball and light signal. Incidentally ‘ball signals’ continued in use in North America and led to the expression ‘high ball’ for a train running at speed. These remained in use in America until 1960.

There were many types of fixed signals, but the semaphore arm adopted from Admiralty for telegraph use became the preferred version. The first appeared on the London and Croydon Railway in 1841, with danger and ‘stop’ (horizontal) and all clear (lowered). These were generally situated on stations, or on the roof of the signal boxes, once they were introduced. With increased speeds, an auxiliary or distance signal was introduced on the approach to warn drivers if the stop signal was at danger. This became known as the distant signal.

In terms of colours, for nighttime working, both white and green were used for clear, with white having a higher status than green (which was used for ‘proceed at caution’ for distant signals). In the UK, green finally became the standard for clear in 1898, but this took longer in other countries.

The signal mechanism was weighted to ensure the arm would return to danger if a breakage occurred and this ‘fail safe’ principle has been used in signalling engineering ever since. In early signalling, a signal only displayed ‘stop’ when a train, junction or other obstruction had to be protected. At other times they would be set to ‘clear’, and the slotted post signal was used, which enabled the semaphore arm to be dropped inside the post out of sight when at clear. However, a serious accident on the Great Northern Railway (GNR) at Abbots Ripton in 1876, with snow and ice holding the arm inside the post, led to the abolition of slotted post signals and leaving signals at clear.

Detonators, to warn drivers of restrictive signals on foggy days, were introduced in 1841. These consisted of a small circular container with a detonating compound inside which is fastened to the rails by lead clips. By 1844 such fog signals were in widespread use. It wasn’t until the introduction of powerful colour light signals in the 20th century that ‘fog men’ were no longer required. 

The time interval system was based on simply allowing sufficient time for the preceding train to have left the section and giving permission for another train to enter the section, but without any knowledge of whether it had done so. So, trains failing in the section for any reason were not protected and accidents occurred. This required a safer way of controlling the movement of trains.

Electric telegraph

A solution was the electric telegraph. William Fothergill Cooke and Charles Wheatstone obtained a patent for their electric telegraph on 10 June 1837.  In 1842, Cooke wrote the pamphlet ‘Telegraphic Railways’ recommending “block signalling” in which track was divided into blocks or sections into which only one train may enter, with their movement in and out of each block monitored electrically. 

The introduction of the signal boxes enabled signals and points to be controlled from one place via wires and rodding. This removed the need for people walking about on the track to move points and operate signals. Centralisation of these activities was more efficient and safer, and it also facilitated the introduction of ‘interlocking’.

In 1843, Charles Gregory at Bricklayers Arms Junction created an embryonic interlocking so that conflicting signals could not be operated at the same time. John Saxby obtained the first patent for what we recognise as interlocking and is often referred to as the ‘father of interlocking’ signals into a single locking arrangement, and is believed to be first to use the interlocking.

The early mechanical interlockings were designed to a defined set of principles, which many still apply today. There were many competing designs of mechanical interlockings, which while similar in principle the detailed designs varied and there were many patent infringement cases in the courts.

For signalling on single lines, space interval was initially used with only one train occupying the line between any two passing places. This was enhanced by providing a physical device (baton, staff, tablet, token) for each section of single line.

Communications and the electric telegraph again provided a safer way of working and a solution to the problem of any imbalance of train movements in each direction. Instruments or token machines were provided and connected by electric wires, such that only one ‘device’ could be out of the instruments at any one time. After withdrawal of a token from one machine, a second could not be obtained from any other instrument until the one already drawn was returned to an instrument.  The first such electric token system was invented by Edward Tyer and patented by him in 1878.

Signal boxes

These were usually two storey buildings, with the signaller upstairs and the interlocking downstairs. The signaller would operate large levers, with separate ones for each signal and set of points. On the ground floor of the signal box, underneath the levers, was the interlocking arrangement of metal bars connected to the levers, with other bars at right-angles to the first. This was arranged to prevent the signaller from moving a lever, unless other levers were in the correct position. For instance, a signal lever could not be moved to allow a train to move, unless the relevant points levers were in the correct position.

Signalling engineers later introduced electro-mechanical locking in combination with the mechanical locking, to prevent a lever from being moved unless its electric lock was energised. This required other levers to be electrically proved to be in the correct position. Other controls were added to the lock circuits to improve safety, such as relevant ‘track circuits’ to be clear before a signal or set of points could be moved.

Safety features were also added to the block systems that controlled the movement of trains between signal boxes, to prevent a signaller from sending a train from the area they controlled to that of the next signal box unless it was safe to do so. Mechanically interlocked signal boxes are still in use and, for example, the Monks Sidings signal box interlocking and frame installed in 1875 near Warrington are still in use today, 150 years later.

Track circuits

The track circuit was invented by William Robinson in 1872, an American civil engineer. The principle is that the connection of the rails by the wheels and axles of trains makes an electrical circuit. This circuit is monitored, typically by relays, to detect the absence of trains in a fail-safe manner. However, any power or equipment failures ‘stop the job’ and are disruptive to railway operations.

It is believed that the first use of track circuits in the UK was in 1886 at St Paul’s station on the London, Chatham & Dover Railway. Like the block telegraph, track circuits were often only initially used at high-risk locations, however accidents caused by signaller error at Hawes Junction in 1910 and Quintinshill in 1915 (226 dead, 246 injured), led to the adoption of the track circuit as a safety device to help remind signallers that a train was present when out of sight.

The need for track circuits became even more important when the activation of the track relay was used to control electric locks on the relevant levers to prevent clearance of signals, or movement of points if a train was present. 

Railway electrification caused a problem with the track circuit sharing the rails with the traction return current and the track circuit’s insulated joints creating a conflict with traction current. As early electrification schemes used Direct Current (DC) distribution it was not possible to use the simple DC track circuit.

A solution to the problem was the Alternating Current (AC) track circuit, with the first in America in 1903. DC traction current could not falsely energise an AC relay, so this method became the preferred method of track circuit operation after 1909. The ‘impedance bond’ was also developed, to provide a low resistance path for the DC traction current, but high impedance to the flow of the track circuit AC.

Throughout the last 200 years there have been huge technical developments in railway signalling, with improved safety, reliability, and performance. This article has only touched on some of them. The changes and improvements have often occurred as a consequence of accidents, inquiry recommendations, or through regulation. However, it is the ingenuity and dedication of engineers who have always adopted the latest technology to improve railway signalling.

Image credit: Westinghouse Brake and Signal collection/Chippenham Museum