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Evolution of signalling control

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Network Rail is currently developing the fourteen Rail Operating Centres (ROCs) which will eventually control the entire national network. Meanwhile, the Hertford National Integration Facility (HNIF) project is under way to further develop the European Train Control System (ETCS) already piloted on the Cambrian lines at Machynlleth. Writes David Bickell

This article is a look at how the mainstream systems in use today have come about. Alas, some readers may be struggling with the acronyms. Is it an IECC or MCS? Can’t tell the difference between SSI and CBI? Where can you find a POSA or an ARS?! Keep reading as we explain how particular products fulfil the functional requirements for the safe signalling of trains.

Signaller Interface

The lever frame was invented in the 19th century to enable clusters of points and signals to be controlled from a single point – the signal box. This saved labour and speeded up train dispatch by obviating the need for the many men needed to move points individually and hand signal trains on the ground. Metal rodding was used to connect the levers directly to point switch blades and metal wire runs connected levers to signal arms. The length of these runs was limited by the physical effort of the lever pull, thereby necessitating the provision of many boxes to control a line. Large stations would need several boxes.

Many of these mechanical boxes are still in service today, such as Heaton Norris Jcn on the West Coast main line (WCML), their longevity being due to the highly robust and reliable nature of the equipment. Maintaining this mechanical equipment is a dying skill and boxes with small areas of control are labour- intensive and unsuitable for the efficient regulation of trains.

The invention of electricity enabled the power operation of points and signals. With this came the development of the panel signal box. Various configurations of switches and buttons were tried out but the method that became standardised for all large signalling centres built from the 1960s onwards is ‘Entrance- Exit’ – dubbed ‘NX’ because 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 (or buffer stops for a terminal line), known as the Exit (X) of the route. The interlocking checks the availability of the route, calls and locks points to the required positions, and displays a proceed aspect to the driver. Confirmation is given to the signaller in the form of a line of white ‘route’ lights along the track diagram and the signal repeater light changing from red to proceed (the actual light is green for main running signals although a signal may be showing yellow or double yellow. White is used for a shunt/call-on proceed aspect).Sig04 [online]

The white route lights change to red with the progress of the train.

Many NX panels are still in use controlling the national network such as Wembley Main line and Kings Cross at the southern ends of the WCML and ECML respectively. One of the disadvantages of a panel is the large amount of fixed hardware used – buttons, switches, indicator lights and tailor made faceplates depicting the track layout controlled. Any subsequent changes to the layout involve significant design work and hardware alterations. Whilst the new island platform at Cambridge was successfully added to the NX panel there, modifying a panel would be extremely challenging to cover significant layout changes such as those taking place at Reading.

Electronics arrive

With the advent of electronic interlocking and computer display technology, British Rail Research (BRR) developed a Visual Display Unit (VDU) interface with which to control trains as an alternative to the panel. The signaller display is a sub-system of what is known as the Integrated Electronic Control Centre (IECC). This may sound generic but, under BR, it was never envisaged that there would be competing products. IECC is actually a specific product supplied, post privatisation, by the company now known as DeltaRail.

The display system, or ‘workstation’, is designed to replicate all the functions of the NX panel. As there are no buttons, routes are set by using a tracker ball to position the cursor over the entrance signal icon, then pressing the ‘set’ button, followed by the same process for the exit signal. The first installations, commissioned in 1989, were at Liverpool Street and Yoker (Scotland).

After privatisation, with BRR sold off, Railtrack wanted to introduce competition into what it saw as a signalling cartel.

Accordingly it cast the net around the world for suitable products. First to come was the then Vaughan-Harmon (now GE Transportation Systems or GETS) Modular Control System (MCS) VDU workstation for the Norwich to Cromer resignalling. GETS has done well with subsequent contracts for workstations around the county including control centres at Rugby and West of Scotland.

Invensys (formerly Westinghouse) also has a popular workstation product called ‘WESTCAD’, installed in East and West Midlands and South Wales centres. Two other VDU interface products are in use on the network though these are to some extent non standard and work only with the interlocking products from the respective companies. These are Siemens’ VICOS at Bournemouth and Havant and the Ansaldo STS ‘ACC’ system at Manchester South with an ‘RTC’ variant at Machynlleth. DeltaRail has recently introduced the new platform of ‘IECC Scalable’ (described in The Rail Engineer Issue 92 – June 2012) as an effective solution for all sizes of signalling scheme.

The interlocking

This is the centre-piece of the fail-safe signalling system. It receives requests from the signaller to move points, set routes and clear signals and executes only if safe to do so. It thus ‘knows’ the current state of the railway ie points/routes locked, trains in section etc. some on trunk routes such as at Shrewsbury Severn Bridge Jcn.

Then came the relay interlocking. This used large quantities of the BR930 relay, a bulky unit approx 2”x4”x6”, performing the same logic as a lever frame. Relay interlockings are usually located near clusters of points and signals to minimise the length of cable runs. Sig05 [online]They may be located many miles from the controlling signal box and are typically linked by a non-vital electronic timesharing data- transfer system called Time Division Multiplex (TDM). There is a BR standard ‘free wired’ (ie tailor made for a particular track layout) relay interlocking.

With the building of large NX panels, manufacturers invented ‘geographical’ relay off-site. The units would be delivered to site and connected together by multicore cables in similar configuration to the track layout being controlled.

Companies producing these products included Westinghouse (Westpac), GC-General Signal (GEC-GS geographical) and AEI-GRS. Examples of these types are still in service today. The last surviving relay interlocking south of Rugby on the WCML is the AEI-GRS geographical at Kings Langley, controlled from Watford Jcn NX panel. Relay interlockings can be controlled from NX panels and some VDU interface products.


The size of the BR930 relays meant that interlockings require significant investment in bricks and mortar! Once again, BRR was ahead of the game. It developed an electronic equivalent of the relay interlocking known as Solid State Interlocking (SSI) – another generic sounding title that was actually a specific product. BRR designed the system, the hardware was manufactured by Westinghouse (now Invensys) and GEC-GS (now Alstom) under a tripartite agreement. Just one rack of electronic modules replaced a large room full of relays.

The first SSI installation was commissioned at Leamington Spa in 1985, controlled from an NX panel. Over time, SSI has proved to be highly successful, being deployed extensively on the national network with considerable export sales.

Storm clouds gathered over SSI when Railtrack took over the infrastructure. SSI was perceived as outmoded – ‘old BR’. The future was with the Computer Based Interlocking (CBI – generic term). Actually, SSI is a CBI, but SSI was apparently dead in the water. The new CBI kids on the block were GETS with the ‘VHLC’ for Norwich to Cromer, Ansaldo STS with ‘ACC’ for Manchester South, Siemens with ‘SIMIS-W’ for Dorset Coast, and Adtra

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  1. Where’s the rest of the article? It finishes abruptly without even a period. Also, the locations where the photos were taken would greatly help.

  2. SSI Smartlock 400 is the preferred (2013) option, as Maxand quotes the end of the article appears to be missing. Signalling Solutions completed the Norwich – Ely using SSI and Modular Signalling


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