As part of the 200-year anniversary of the opening of the Stockton & Darlington Railway on 27 September 1825, in Issue 213 (Mar-Apr 2025) we discussed the early developments in signalling during the 1800s. We now take a look at developments in signalling since 1900.
Train detection
The Track Circuit (TC) is an electrical device that proves the absence of a train in a fail-safe manner. The first TC’s used Direct Current (DC) to activate a relay connected between the rails some distance away. A train located on the track short circuits the relay and indicates a train is present. The Alternating Current (AC) TC was introduced in 1909, primarily because the DC traction current used to power trains could not falsely energise a TC AC relay.
Track circuits also allowed Track Circuit Block (TCB) to be introduced. The train detection function of the TC provides a continuous indication of the position of trains, so signallers didn’t have to visually observe that every passing train is complete with its tail lamp to confirm it had left the previous block section.
A communication link between adjacent signal boxes provided a means of passing train descriptions using more sophisticated train describers. Train describers were also developed to store trains’ class and routeing details and display these to the signaller.
Since there is no need to locate a signal box at the extremities of every section, any number of consecutive sections could be placed under the control of the same signal box.
Coded track circuits
Track circuit development also played a part in the introduction of train protection. It was found that a coded AC TC could be used to control a display in the train cab and, in 1933 in Philadelphia USA, coded TCs were used for both the control of signals and train in-cab displays. Coded AC TCs could also operate over greater distances than a DC TC. The Western Region of British Railways used coded TCs in the late 1940s and 1950s as these were able to tolerate the low ballast resistance with the use of through-bolted fastenings.
A problem with TCs is that any rail contamination may cause a high resistance / impedance ‘block’ to the TC current and result in a wrong side failure, with trains ‘disappearing’. This became even more of a problem with the introduction of lighter rolling stock. Poor ballast conditions can also cause track circuit failures. An alternative form of train detection to address these problems is the axle counter. By 1904, one manufacturer had introduced a system which displayed the count via a needle stepping around a pointed dial in a case in the signal box.
However, it would be some time before the axle counter was made a reliable form of train detection. The first axle counters to be fitted in service in the UK were at Glasgow Queen Street station in 1967 and today axle counters with no moving mechanical parts are widely used for train detection purposes.
Axle counters do not require insulated joints, unlike many track circuits, and allow both running rails to be used for traction current return in electrified areas. However, a track circuit provides continuous detection and will recover from a power failure to indicate the presence or otherwise of a train, but the axle counter will not have the ability to safely determine the occupancy of any section after a power failure. When this occurs, a reset and restoration process must be followed to check the section is clear before normal working is resumed.
Colour light signals
Colour light railway signals were first used in the USA in the early 1900s and on the UK’s Liverpool Overhead Railway in 1920. Rail led the road industry, as colour light road traffic signals in the UK did not appear until 1926. Colour light signals could be seen far better and further away, especially in poor weather. This improved safety, and fog signallers used during fog and falling snow were no longer required.
The need to provide route information to drivers at higher speeds saw the Junction Route Indicator (JRI) introduced, first at Thirsk in the north east of England in 1933. Initially comprising an angled neon strip, the final design used either three or five white lights above the main aspect at an angle to represent the direction of the turnout. These became known as ‘feathers’.
Tungsten filament lamp technology was eventually overtaken by the use of Light Emitting Diodes (LEDs). The improved intensity and longevity, together with immunity to phantom aspects caused by reflected sunlight, provided both a safety and economic benefit – particularly in respect of routine lamp changing and the saving in train delays caused by lamp failures.
Train protection and warning
Interlockings and other systems can provide protection from signaller error, but there was also a need to prevent trains being driven too fast or passing signals at danger. Any train protection system needs a link between the fixed signalling at the track side and the train which is moving. Many forms of train protection have been developed over the years to either advise the driver in the cab or to enforce a stop signal.
The Great Central Railway (GCR) had experimented with a track-circuit-based system in 1903, and in 1915 trialled another mechanical system known as Reliostop. Mechanical actuation and electrical contact was used in the GWR Automatic Train Control (ATC) system and the French ‘Crocodile’ which was developed as early as 1872 and is still used in France. The Crocodile is the name given to the ramp placed between the rails through which a brush on the locomotive picks up current. One polarity signifies the signal is at clear, the other that it is restrictive.
The GCR and Great Northern railways also experimented with the Boult magnetic, non-contact arrangement. This system laid the foundation for the eventual development of the Strowger-Hudd and subsequent BR Automatic Warning System (AWS).
Train protection can consist of a warning to the driver and a train stop provided at a stop signal or buffer stop. Ideally both should be provided but, for economic reasons, only the warning element was provided in the UK systems. In Europe, however, systems were developed during the 1930s that encompassed both elements.
Train protection in its simplest form is a warning system to alert drivers of the need to take action. It then migrates to simple train stops which activate the brake on passing a signal at danger. Later developments have been to detect overspeed events and initiate the brakes. Today, systems can continuously ensure the train remains within a safe speed envelope.
Integrated signalling
The adoption of power operation brought together the lever, circuit controller, and locks (both mechanical and electric) into one unit. The miniature lever frame was a step from full-size mechanical levers to control panels. Power operation meant manual workload was reduced and one box could take over the work of several boxes, and on plain line automatic TC Block (TCB) signalling enabled the abolition of intermediate boxes.
In 1927, the GWR introduced a route setting installation at Newport in Wales. A lever was used to control a route, setting the points and then clearing the relevant signal. Eventually the route setting concept became the preferred method for the next generation of relay interlocking.
Relays also enabled signals and points some distance from the signal box to be operated using local power, switched by a relay at the end of a circuit controlled from the lever in the signal box. The operation of the remote equipment could also be sensed and an electrical circuit returned to a relay in the signal box, providing indications to the signaller.
In the 1950s, while electromechanical, power, and relay interlockings co-existed, relay interlocking became the dominant technology. Manufacturers developed smaller relays inserted into a plugboard, which were tested and sealed in a factory environment and could be easily replaced if faulty. This also saved space and enabled the relays to be installed after the plugboards had been wired and tested.
With geographical signalling, factory-wired relay sets were designed for common functions. These were connected together by plug-coupled cables in accordance with the geographic relationship of the objects on the railway. So any possible route through the layout was catered for, rather than having to be designed and built separately. If a particular route was not required this had to be suppressed and it was also necessary to add ‘free wired’ circuitry for special conditions. An entire relay interlocking could be free wired for smaller schemes. This became known as Route Relay Interlocking (RRI).
SSI and CBI
Electronics and microprocessors were introduced in the 1980s to replace relay interlocking, although the extensive use of software driven systems has taken an awful long time compared to other industries, as the safety validation of software interlockings isn’t easy.
British Rail Research developed an electronic equivalent of relay interlocking known as Solid State Interlocking (SSI) with the hardware supplied by Westinghouse Brake & Signal (now Siemens) and GEC-GS (now Alstom) under a tripartite agreement.
The first use of SSI for interlocking purposes was in 1984 at Dingwall in Scotland, as part of the Radio Electronic Token Block (RETB) system to replace key token signalling with electronic tokens issued over radio into the train cab. SSI was then used for the interlocking functionality and controlled trackside signals and points at Leamington Spa in 1985. SSI was also applied to metro-style operations from an early stage, being used for the original signalling on the Docklands Light Railway in London in 1987. Today, modern Computer-Based Interlocking (CBI) systems are provided by a number of suppliers and are the mainstay of most railway signalling.
The most popular configuration is the two-out-of-three (2oo3) architecture. With this, in the event of the loss of one channel not impacting the operation of the railway, the remaining two channels continue in two-out-of-two mode until the faulty module is restored. SSI adopted this architecture, as did other designs, but variations using 2oo2 and even single channel hardware with diverse software and data running in parallel have been used. The availability being enhanced by a hot-standby duplicate if required.
RETB and radio produced an affordable solution for lightly used lines. The RETB system is currently being renewed with modern electronics, but still using the same principles devised 40 years ago. Modern signalling systems which uses the skills of both signal and telecoms engineers include the European Rail Traffic Management System (ERTMS) and Communication Based Train Control (CBTC) for metros. These systems provide in-cab signalling, Automatic Train Protection (ATP), and Automatic Train Operation (ATO) in various grades.
Signallers interface
Various configurations of switches and buttons were developed for the signaller interface. However, the preferred method for large signalling centres from the 1960s onwards was the ‘Entrance – Exit’, called ‘NX’, is to clear a signal the signaller presses and releases a button at the Entrance (N) of the route, followed by another button at the next signal ahead, known as the Exit (X) of the route.
One of the disadvantages of a panel was the large amount of fixed hardware and any changes to the layout involve major design and hardware alterations. So the next development was the Visual Display Unit (VDU) interface. Known as the ‘workstation’, these were designed to replicate all the functions of the NX panel, but more flexibly. Routes are set by using a keyboard, tracker ball, or mouse to position the cursor over the entrance signal icon, then pressing the ‘set’ button, followed by the same process for the exit signal.
Metro signalling
Metro railways have always been at the forefront of signalling technology, with the requirement to provide intense services with short headways. Having the same trains on a route and no need for interoperability helps, and London Underground was the pioneer in the development of ATO. Experiments were begun in 1963, with a trial in 1964 on the Central line shuttle between Woodford and Hainault. This resulted in the Victoria line being made the first full scale automatic railway in the world.
For ATO and ATP a connection is required from the track to the train. Early systems used coded track circuits or communication ‘loops’. Wi-Fi systems have been used for metro CBTC systems but, similarly to ETCS, radio-based communication is now the preferred technology.
Future signalling
System suppliers are starting to combine the functions of interlocking and track-train messaging within the same system and there is already talk of the next generation of signalling combining CBTC and ERTMS. Artificial Intelligence (AI) will provide solutions for such things as testing and much more.
Cloud technology is another area which could deliver benefits, such as lower cost, flexibility, and resilience. Maintaining cyber-security to ensure safety and reliability is already important and will be increasingly so. Signalling is also likely to move towards distributed architecture, with some functions shared between the train-borne and trackside systems. This is already happening with CBTC and ERTMS.
Trains may in some situations initiate the setting of the route ahead and protect themselves using train-to-train communications, with trains becoming far ‘smarter’ rather than simply responding to movement authorities issued from trackside signalling.
A major challenge will be equipment obsolescence. People generally change their cars every few years and some upgrade smart phones every year. Signalling installations, however, have typically lasted for 40 years or more (much more in some cases). In the future, signalling renewals are likely to be required more often.
The author appreciates David Fenner’s help with this article.
Image credits: Credit Westinghouse Brake and Signa