Signalling relays and their place in the development of modern signalling M.P.White Q4 2010 Introduction Despite the introduction of Solid State and Computer based interlockings, in the mid 1980s, over twenty years later vital signalling relays are still in production and continue to be an important component of modern signalling schemes. This article will hopefully explain this apparent paradox. What is a relay? According to the Oxford English dictionary, a relay is “an electrical device that opens or closes a circuit in response to a current in another circuit.” It would be more accurate to say that a relay opens or closes several circuits in response to a current in another circuit. A relay is in essence an electro-mechanical switch. By energising/de-energising one relay, several other relay circuits can be controlled by contacts of that relay. When the relay coil is energised, this creates an electromagnetic field that attracts the relay armature to the coil. Attached to this armature is a mechanism for either opening or closing the relay contacts that are electrically independent from the input voltage to the relay coil. The relay can in turn control (or switch) other circuits. These other circuits that are switched, may or may not be at the same voltage as the relay coil whose contacts are doing the switching. In this way, complex functions such as interlocking between points and signals can be performed by a relay interlocking consisting of perhaps several thousand relays in the largest installations. Relay circuitry can be configured to provide all logic functions such as AND, OR, NAND, NOR and any combination of these. Most relays have two types of contacts. One set of contacts are “made” or closed when the relay is energised. These are known as “Front Contacts.” Conversely these contacts are opened or broken when the relay is de-energised. Back contacts of the relay on the other hand are “made” (closed) when the relay is de-energised and broken (open) when the relay is energised. Terms “Front” and “Back” contacts originate from the days of large “shelf” type relays. In these, the terminals for connection to external circuits were located on the top of the relay. On these relays the contacts that are closed when the relay is energised are located at the front of the relay-Hence the term “Front Contacts”. The contacts that are closed when the relay is de-energised are located at the back of the relay-Hence the term “Back Contacts.” Note however that physically, the front and back contacts are similar.

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On a plug-in relay, such as BR930 series, the original derivation of the terms “Front” and “Back” contacts is no longer relevant. However, by convention, the terms remain to describe the contacts that are closed when the relay is energised and de-energised respectively. Different types of relays have different combinations of Front (F) and Back (B) contacts, dependent on the main application of the relay. E.g. in the BR 930 series of relays, used on Singapore SMRT, the QN1 relay which is the basic neutral relay in the series, is commonly available in either 12F 4B or 8F 8B configuration, dependent on the contact requirements of the circuits in which the relays are used. These relays are supplied by Westinghouse Rail Systems in the UK. Relay types The basic signalling relay is known as a Neutral Relay. Variations on this include “Slow to Pick”, “Slow to release”. In these, the operational pick/release times are increased, by the addition of a metallic “slug” at either end of the relay armature. It should be noted that the addition of these “slugs” also make the relays “AC immune”. That is they will not operate if AC voltage is applied across the coils. This is important on railways that have AC traction supply voltage, to prevent relays incorrectly energising due to voltage induced from AC traction supply. (Not applicable to Singapore). Note that techniques to change the relay operating times, such as addition of diodes or capacitors across the relay coil, are not usually used in vital signalling circuits, due to potential for wrong side failure. They can be used in non vital circuits, where there are no safety implications if the relay timing reverts to that of a neutral relay. Another type of relay commonly used in interlocking circuitry is the magnetically latched relay. This type of relay has two windings. These are known as “Pick Up” and “Release” coils. Once the “Pick Up” coil of a latched relay has been energised a permanent magnet will maintain the relay in the “latched” state, until the “Release Coil” is energised. The “Release” coil is wound in the opposite direction to “Pick Up” coil, such that when it is energised, an electromagnetic field is created that is in opposition to the permanent magnet. When the “Release Coil” is energised, the relay unlatches. By use of latched relays, the state of the signal and point interlocking can be maintained following a power failure. This is particularly important in the case of points, to ensure that they remain in their last set position and don’t move following restoration of power, unless legitimately called to, by the signalling system.

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“Biased” relays are similar to neutral relays, except that they will operate only if the voltage polarity is correct. By use of a pair of biased relays, two positive controls or indications can be transmitted over one pair of wires. In this way, the amount of cabling used for lineside relay circuits can be reduced. Because most of the lineside signalling equipment on Singapore NS-EW MRT is fed a relatively short distance, directly from the nearest Signalling Equipment room, there isn’t generally a need for “Biased” relays. The exceptions are at entrance to Bishan Depot from Ang Mo Kio Direction and entrance to Ulu Pandan Depot from Jurong direction. The point detection circuits at these locations are fed back to relevant SER, using biased relay circuits. In addition the signal aspect repeater circuits also use biased relays at the respective relay room. At Bishan Depot, biased relays are used on point control/detection and signal repeater circuits to/from Bishan Depot Tower Relay Room and Bishan Depot East Relay Room The above covers the main types of signalling relays. There are others for special applications.

Photo 1-Westinghouse Q relay (BR 930 series) in use on Singapore MRT

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History 1830s-1870s The early railway signalling systems were purely mechanical, with no safeguards, apart from the mechanical interlocking systems between points and signals, to prevent conflicting signals being cleared simultaneously, or points being moved once a signal to route a train over them had been cleared. Note that once a signal had been replaced to danger after the passage of the train, there was initially nothing to prevent the points ahead of the train being moved. Eventually mechanical “depression bars” at the points were added to prevent this happening. The only electrical equipment in the signal boxes of the mid 19th century were the battery operated block instruments used to allow operators to communicate by bell codes, to signal trains between each other. These block instruments were similar to early telegraph instruments. 1870s The first relays to be introduced into signalling systems were probably track relays associated with track circuits. The track circuit was invented in the United States in 1871 and introduced into UK a few years later. The track circuit enables the position of trains to be detected within a defined area. A track circuit relay is normally energised when there is no train present on a given piece of track. The track circuit relay is de-energised when a train is present, or there is any failure in the circuit, such as a blown fuse or detached wire. i.e. a track circuit actually proves the absence of a train in a defined area. In this way, the fail safe principle of railway signalling is maintained. i.e. the failure of the track circuit will revert the signalling system to its most restrictive state. The first track circuits were used simply as reminders to the signal men (operators) at key installations. i.e. there was initially no interlinking between the track circuits and signalling controls. 1870s-1920s The widespread adoption of the use of track circuits, after their invention in 1871 was initially slow. However this changed after of two particularly bad railway accidents in UK at Hawes Junction on the Midland Railway in 1910 and at Quintinshill, just north of the Scottish border at Gretna in 1915.1 To this day, the accident at Quintinshill remains the worst in UK railway history, with over 200 fatalities. The wooden construction of coaches and gas

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lighting was a dangerous combination that contributed to the number of fatalities.

Photo 2-Quintinshill signal box in early 1970s Both these accidents were due to simple error on the part of the signalmen, in that a train was signalled into a stationary train whose presence had been forgotten. In both cases, the accidents could have been avoided by the use of track circuit controls. Following these accidents, more widespread use of track circuits was introduced, including the interlinking between track circuits and signal controls. Electric locks were introduced on the mechanical signal levers, e.g. to prevent a signal clearing, or point moving, if a particular section of track was occupied by a train. These locks would only be released if the relevant track circuit was clear. Once track circuits were used to lock points, the mechanical “depression bars” that had previously done this job, were no longer needed. The locking of points by track circuits was extended to prevent movement of points, if “foul” track circuits were occupied. This additional protection was first introduced around 1920’s to 1930’s.2 In addition, the block instrument controls were interlinked with signals controlling entry to block sections, to prevent a train entering an occupied block. It was the introduction of these safeguards that brought about more widespread use of signalling relays, in the first part of the twentieth century.

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However, the Signal Engineers of the day were conservative in nature and the vital interlocking between points and signals remained mechanical, with the relays acting an interface between track circuit and block controls and the electro-mechanical locks. The number of relays was minimal in such installations. The introduction of track circuiting was a gradual process and at first was restricted to mainline major stations and junctions. The majority of the railway between adjacent interlockings/stations remained non-track circuited, with the “Block system” ensuring the safe separation of trains. This was of course heavily dependent on human vigilance. At this stage, the majority of signals and points remained mechanically operated. It was only with the gradual introduction of electric colour light signals and electrically operated points that more relays were introduced into signalling systems. In the UK, the main areas where this first happened were the busy commuter routes south of London and the London Underground system. (LUL). The introduction of colour light signals in conjunction with track circuits enabled signals to be automatically replaced to red, after the passage of a train. In addition, on plain line, signals could be operated automatically, with no operator intervention. Such features reduced the operator’s work load. The first colour light signals introduced on LUL were simple 2 aspect signals. On the mainlines in 1920’s the first 4 aspect signals were introduced.3 These introduced more relays into the signalling system. In the 1920s a major development was the introduction of interlockings controlled by miniature lever frames. The interlocking between the levers was initially still mechanical, but interface relays operated from contacts on contact bands that either made or broke contact depending on the position of the lever. In this way, the mechanical linkage between the interlocking and the signals and points at the trackside had been removed. The main disadvantage of maintaining the mechanical interlocking between the miniature levers was that there was a limit to the size of interlocking that could be controlled by one installation. In some of the larger installations in London, this limit was being reached, due to the complexity of the mechanical interlocking required and the difficulty in carrying out modifications.4 The London Bridge interlocking installed by Westinghouse Brake and Saxby Signal Company in 1927, contained 311 mechanically interlocked levers.5 The next development was to replace the mechanical interlocking between the miniature levers with all electric locking.

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This was first done in UK in 1929, at North Kent East Junction, just south of London.6 Electric locks on the levers prevented the reversal of that lever e.g. to clear a signal, unless the relevant conditions such as track circuits clear and points correctly detected and locked were satisfied. The adoption of all electric locking on miniature lever frames enabled larger areas to be controlled from one interlocking. Another advantage of all electric locking on miniature lever frames was that at large installations, the lever frame could be split into two sections. The Crewe North Junction installation was an example of this with the lever frame split into two back to back sections, with one used for control of up direction trains and the other for down direction trains. Of course the circuitry that was introduced to interface between the lever locks used additional relays, compared with the previous mechanically interlocked systems.

Photo 3-Crewe North Junction (Westinghouse Style L lever frame)

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1930s A fair number of all electric miniature lever frame signalling installations were built in the 1930s in UK. These were designed and installed mainly by Westinghouse Brake and Signal Company. (Style L frame), Siemens and General Electric Company (SGE) and General Railway Signal Company (GRS). Most electric miniature lever frames in UK replicated mechanical interlocking frames, in that one lever was provided for one function. In continental Europe, miniature lever frames were developed in which operation of a single lever operated a complete route. The operator wasn’t required to operate each set of points individually. This system wasn’t widely adopted in UK, though an example was at Newport in South Wales. Here the Insell Ferreira Route Signalling System was used. The disadvantage of route lever frames was that on larger schemes, more levers and relays were required, compared to a lever frame with a separate lever for each signal and point. Despite the advances described above, the majority of the world’s railways remained mechanically signalled. Limited adoption of safety improvements such as track circuiting and interlinking with signal and point controls proceeded in the 1930s at the busiest stations and junctions. Another major safety improvement to mechanical signalling that took place in the 1930s in the UK was the introduction a feature in the UK known as “Welwyn Control”. This added additional relays to the signalling system. This was an improvement to the control that prevented signals controlling entrance to block sections clearing unless the adjacent signal box had given a “Line Clear” on his block instrument. This was introduced after a serious train collision at Welwyn Garden City, in 1935. In this accident a signalman inadvertently allowed a train to enter a block section between adjacent signal boxes that was still occupied by a previous train.7 The Welwyn control prevented a signalman giving a “Line Clear” release to the signal box in rear, until the previous train had arrived and occupied and cleared the “berth” track circuit, and the “Home” signal had been replaced to danger. An earlier system to interlink signals with block controls was the “Sykes Lock and Block” system, patented around 1875. This was used extensively on the former Southern Region of British Railways. Before the widespread adoption of track circuits, lineside electromechanical treadles were used in this system to detect trains. However the Sykes Lock and Block system wasn’t fool-proof, as was demonstrated at the serious crash between Purley Oaks/South Croydon, south of London in 1947, when the system was incorrectly overridden by the signalman.

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In the 1930s, with the additional safeguards described above, mechanical interlocking probably reached its full potential. (Though it should be noted that some new all mechanical signal boxes were still being installed in UK up until the early 1970s, in isolated cases). Other improvements introduced around the 1930s time were the introduction of colour light “Distant” signals on high speed main lines. (These were signals that gave advance warning to a train driver of the status of the stop signals at the next signal box). By converting these signals to colour light operation, even when operated from a mechanical signal box, they could be located further away from the controlling signal box. The introduction of colour light distant signals reduced the manual effort required by the signalman. (The effort to operate a distant lever controlling a mechanical signal up to 1000m away could be considerable). The colour light distant signals were still operated from the same lever that had originally operated a mechanical semaphore signal, but the lever was fitted with a circuit controller that in turn energised a relay to operate the distant signal. The adoption of colour light distant signals was an important factor that enabled line speeds to be raised on busy main lines such as West Coast Main Line and East Coast main line in the 1930s. Another adoption of colour light signals, in otherwise mechanical areas that started in the 1930s, was the adoption of “Intermediate Block” (IB) signals. On some lines intermediate signal boxes had originally been provided between stations and junctions, solely to break block sections into manageable lengths, and reduce headway between following trains. With the introduction of “Intermediate Block” signals, these intermediate signal boxes were abolished and replaced by two aspect colour light signals. These were operated by direct wire relay circuits from the nearest adjacent mechanical signal box, possibly several kilometers away. Such intermediate block signals were introduced on lines with a fairly high level of traffic, but where there was a long distance between adjacent stations and junctions. The West Coast main line between Preston and Carlisle in England, is a good example of where Intermediate Block signals were extensively used. Full track circuiting was provided between the starting signal of the controlling signal box up to the Intermediate block Stop signal. In addition, the provision of a separate track circuit at the then standard overlap length of 440 yards beyond the IB stop signal was provided. In this way, two trains could be safely signalled between adjacent signal boxes. (In very long sections between adjacent signal boxes, additional IB signals could further increase the number of trains between adjacent signal boxes).

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So yet more relays were being introduced into what was still predominantly a mechanical signalling system. At the same time, the productivity was being increased, by the abolition of the intermediate signal boxes. Note that in exceptional cases, mechanical Intermediate Block signals were introduced, but these were the exception. An example of mechanical Intermediate Block signals, could be found at Standish Junction, north of Wigan on the West Main Line in UK. These were abolished in 1972, when the West Coast Main Line was re-signalled using multiple aspect colour light signals and continuous track circuiting. Beyond all electric miniature lever frames, the next major development was the introduction of all relay interlockings. In these the miniature levers operating the signals and points were replaced by control panels fitted with buttons or switches to control the system. Various methods of operation were developed, including “One Control Switch,” “Individual Switch” and “Entrance-Exit” working. Due to it’s simplicity of operation and compactness of control panels, the “Entrance-Exit” system was widely adopted from the early 1960’s onwards. The local control panels situated in Passenger Service Centres on NS-EW lines in Singapore use the “Entrance-Exit” method of operation. In addition the three depots on NS-EW lines use this system, first developed in the United States and introduced into the UK in the 1930s at Brunswick near Liverpool.8 The main advantage of the “Entrance-Exit” method of route setting, is that only two operator actions are required to set any route, regardless of it’s complexity. This compares favourably with some “state of the art” systems introduced more recently in which up to six operator actions are required to set a route! Whilst the “Entrance-Exit” system of route setting simplified the operation of the system, yet more relays were introduced to interface the push button circuitry to the route calling circuitry. The adoption of all relay interlocking, operated from route setting panels enabled larger areas to be controlled from one signal box. Such relay interlockings introduced in the UK in the late 1930s form the basis of modern interlocking practice. (Although of course the interlocking controls have been refined over the years). The move to all relay interlocking, introduced even more relays into the system. It is interesting to note that there were a small number of “Hybrid” signalling installations installed in the UK in the 1930s. Examples of these were Wigan No1 and Wigan No 2 and Wigan Wallgate signalboxes in the North West of England and Shoeburyness in South East England.

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For these installations, the points were operated electrically, but via circuit controllers on levers on a full sized mechanical frame. The signals however were operated from miniature switches located in the correct geographical position on the signal box illuminated diagram. The interlocking between the points and signals was relay based. Electric locks on the point levers driven from relay interlocking circuits, ensured the points could not be moved unless safe to do so. The installations at Wigan were designed and installed by SGE (Siemens and General Electric Company). These “hybrid” installations were not widely adopted. This was probably because greater economies could be made by adoption of a full relay based system rather than a hybrid mechanical/relay system that still had the expense of the mechanical lever frame to operate points. Further signalling development in general in the UK was halted by the advent of the Second World War and its immediate aftermath. The installations at Wigan No 1 & Wigan No 2 were de-commissioned at the time of the West Coast Mainline re-signalling in 1972. The Wigan Wallgate installation reverted to a conventional mechanical interlocking when the original frame was replaced in 1977. This in turn has recently been replaced by a relay interlocking, operated from a conventional Entrance-Exit panel. 1950s In the 1950s, all three technologies described above existed side by side. The majority of UK railways remained mechanically operated, with the safeguards such as interlinking between signal/point levers and track circuit controls and block controls added on the majority of mainline installations. On lightly used branch lines or goods only lines, it was quite common for the signal boxes to have none of these safeguards. Principal stations in large cities were usually signalled from signal boxes equipped with large route setting panels (e.g. York and Newcastle) or miniature lever frames e.g. Crewe North Junction, Crewe South Junction and Waterloo and Victoria stations in London. The design of the relays used in all of these installations hadn’t changed to any great extent in 40-50 years. The largest of these were very cumbersome devices, with the nick-name “fish-tank” or shelf type relays. It was realised that with the more extensive use of relay interlockings in the future, the large size of existing designs of relays would be a limiting factor, due to the space that would required to house these relays in large interlockings.

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Photo 4-Westinghouse Shelf type relay in Crewe North Junction relay room 1960s In the early 1960s several large resignalling schemes were in the planning stage in the UK. (e.g. London-Manchester & Liverpool). In these schemes, large numbers of mechanical signal boxes sometimes dating from 1880s were to be abolished. They were replaced by a smaller number of new signal boxes controlling a number of all electric relay interlockings, in conjunction with multiple aspect signalling and continuous track circuiting. Some of the first installations to be introduced e.g. at Manchester, Wilmslow, Sandbach used “free-wired” interlockings. Wilmslow and Sandbach have only recently been de-commissioned and replaced by a CBI installation. Later installations used the first “Geographic” relay interlockings in the UK. These were based on systems used in Continental Europe. In this system, relays for the main interlocking functions such as signals, points, tracks were packaged into standard units produced in the factory. These were then connected on site with standard multicore cables and a relatively small amount of “free-wiring” to take care of interfacing to trackside and local “peculiarities”, not covered by the standard geographic system.

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The main advantage of Geographic interlockings, was the speeding up of application design and installation on site. The main disadvantages of geographical interlocking was that there was “redundant” wiring and relays in the geographical units, because they were generically designed to provide a wide variety of facilities that weren’t always required. In practice it was also found to be more difficult to implement large scheme changes on geographical interlockings, compared to free wired ones. A number of the geographic installations dating from mid 1960s are still in use at such places as Birmingham New Street, Wolverhampton, though these are now overdue for renewal. The geographic interlockings on the northern part of the West Coast Main Line at Warrington, Preston and Carlisle areas are also still in use. On the Preston scheme at Carnforth, some of the Geographical units were refurbished in 2006 by the original manufacturer and some units replaced by newly built units. The last all new geographic relay interlockings in UK were installed on the Three Bridges scheme in early 1980s. The adoption of continuous track circuiting, in conjunction with multiple aspect colour light signals, enabled an operating system know as “Track Circuit Block” to be widely adopted on new signalling schemes introduced. The main advantage of this was the abolition of manual block working on the newly equipped lines and a further reduction in operator workload. This in turn increased productivity by enabling one operator to control a larger area. With the introduction of track circuit block and continuous track circuiting, the signalling controls could be simplified, e.g. because such features as “Welwyn Release” were no longer necessary. A useful feature introduced in the 1960s was the “Auto Working” feature on selected controlled signals. The operation of this feature eliminated the need to cancel and reset the route, when successive trains were taking the same route. This greatly reduced the operators work load, and wear on the control panel buttons. It was apparent that the proposed large schemes couldn’t be economically implemented using the existing relay designs. The major UK signalling manufacturers, in conjunction with the railway authorities and the Institution of Railway Signal Engineers began preparation of specifications for a new range of miniature vital signalling relays. The result of this was the production of the BR 930 specification for miniature “plug-in” relays. Westinghouse produced their Q style relays to meet the BR 930 specification. The predecessor companies to the present Alstom Company also produced relays meeting the BR 930 specification in UK. The relays typically operate at 50V DC, though 24V DC versions are available for specific applications.

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The initial signalling schemes of the early 1960s utilised miniature plug in relays that were the immediate predecessors to the BR 930 style relays. (eg Westinghouse P and R Style relays). Eventually by the mid 1960s, the BR930 style relay became the standard for new schemes in UK and in other countries supplied by UK manufacturers. These relays represented a considerable improvement on the previous shelf type relays. These improvements can summarised as:

• Greatly reduced size of relay.

• Relay plugged into a separate base, enabling change of relay to be quickly and safely done, without interfering with connecting wiring.

• Reduced cost of relays.

It is a tribute to the original design that the BR 930 series of relays haven’t changed substantially after more than 40 years. It should also be noted that the BR930 signalling relays are extremely reliable devices, with a Mean Time Between Failures (MTBF) of approximately 550 years.9 The Mean Time Between Wrong Side Failures (MTBWSF) for BR 930 Q relays is estimated by Westinghouse Rail Systems to be 6.89 x 109 hours.10 Several thousand of the Westinghouse Q style of relay to the BR 930 spec have been installed on the existing SMRT NS-EW lines in Singapore. They will also be installed on the future Boon Lay Extension line, currently under construction. In total, well over two million Q relays have been made so far and are in service in many countries world-wide. 1970s Major resignalling schemes covering large geographical areas, using all relay based “Geographic” interlockings, operated from large push button “Entrance-Exit” panels were introduced. These included the northern section of the West Coast mainline, (from Weaver Junction to Glasgow, via Warrington, Preston, Carlisle), London Bridge, East Coast Main line (Doncaster, Peterborough and Kings Cross). The London Bridge scheme, designed and installed by Westinghouse Brake and Signal Company was commissioned in 1975. It was at the time the world’s largest largest geographical interlocking, containing some 13,000 relays.11 Relay interlocking probably reached its full potential in terms of development in mid to late 1970s.

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The majority of the schemes described above are still in operation. The adoption of high speed TDM links enabled the area controlled from one signal box to be greatly increased compared to the earlier schemes of the 1960s. The advent of such schemes had been accurately predicted as far back as 1938, by Mr E.W.Challis in his paper to IRSE titled “A comparison between Relay and Electric lever interlocking”.12 Train Operated Route Release (TORR) was introduced for the first time in UK on some schemes. This greatly reduced operator workload and wear and tear on the panel push buttons. Further a field, in Hong Kong, the MTR (Mass Rapid Transit Railway) was signalled using full ATP fixed block system. This used BR930 style relays for interlocking and switching of ATP speed codes. (Equivalent to aspect sequence circuits on a conventionally signalled railway). 1980s The last large all relay based schemes such as Three Bridges on Brighton Line south of London were commissioned. The main development in signalling in the 1980s was the development of Solid State and Computer based interlockings, with the first installation in UK going into service in 1985. In these systems, the interlocking functions that had previously been done using hard-wired relay circuitry were replaced by logic within the computer or processor. One of the main savings promised to be much reduced overall size of equipment and wiring required for a given interlocking. In addition, the amount of lineside cabling could be reduced. Consequently the rooms required to house the equipment could be reduced in size, with associated cost savings. Metro railways generally have small sized interlockings compared to mainline railways in Europe, for example. Consequently, there is little or no saving in room size, except for depot interlockings. The saving in lineside cabling with introduction of SSI/CBI, isn’t usually applicable to Metros, where the majority of trackside equipment is directly fed from the nearest Signalling Equipment Room. (SER). Another advantage of SSI or computer based installations is the reduced on site installation and the fact much of the interlocking testing can be done in a factory environment.

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However on some complex metro systems, the large amount of on-site interfacing testing required from interlocking to ATC and ATS systems, to a certain extent negates the saving that have been made by adoption of CBI. Before SSI was fully adopted, there were a number of “hybrid” schemes in the mid 1980s that used various techniques to reduced the number of relays used in relay based installations. These included ERSE (Electronic Route Setting equipment) adopted by the former Southern Region of BR at Waterloo in London, Brockenhurst, Dover Priory and Salisbury. (The electronics of these ERSE systems were in the process of being replaced in 2006). On the London Midland Region, the new installations at Crewe and Manchester Piccadilly and Guide Bridge used “free –wired” relay interlockings, in conjunction with “Panel Processor” systems. These “hybrid” schemes enabled a reduction in number of non-vital relays used in route selection and panel indications, whilst still retaining the relay interlocking. However, as with previous “hybrid” systems, when the transition from mechanical to all relay based systems occurred, such installations were not widely adopted. The wholesale adoption of SSI/CBI systems for most new schemes was just around the corner. These promised greater savings, compared to “hybrid” systems. In Singapore, the first stage of the MRT opened in 1987 from Yishun to Toa Payoh. This included several upgrades in technology compared with the Hong Kong MTR opened less than 10 years earlier. However, BR930 relays were still used for interlocking and ATP code switching. The SSI system that was adopted in UK around the same time, was initially developed to be a very BR specific system and no modules were available at that time for switching of ATP speed codes. In addition, significant development would be required to adopt SSI to a metro application. SSI was eventually introduced on the Eastern Harbour crossing extension of Hong Kong MTR. This used SSI for interlocking between points and signals, but with the ATP fixed block system, being driven from vital relay circuitry interfacing with the SSI. The “Fixed Block” ATP system used on NS-EW lines in Singapore is a development from earlier ATP systems introduced in UK on Victoria Line and Hong Kong MTR line. These were in turn based on the “Track Circuit Block” systems introduced in early 1960s on BR main lines. Instead of multiple aspect signals along the lineside, the trains receive ATP speed codes that are injected into the running rails. Each ATP speed code takes the form of a specific signal frequency.

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This is decoded by ATP equipment on the train, to instruct the train to drive at a particular speed. This can be without operator intervention, pathing the way for driverless operation. 1990s Major advances in computer technology enabled systems such as “Integrated Electronic Control Centres” (IECC) to be widely adopted in UK. (The first ones had been introduced in the late 1980s). Equivalent systems were developed in the rest of Europe and United States. IECC eliminated the need for the large push button control panels that had been a feature of most new signalling schemes since the mid 1960s. In addition, relays previously required for panel interface of both indications and control switches/buttons could be eliminated. Due to local operating constraints, it was realised that IECC wasn’t suitable for all areas and in particular cases, e.g. at Wembley control centre on West Coast Main Line, just north of London, a conventional “Entrance Exit” control panel was used to control SSI interlockings. In areas where there are a large number of non-timetabled train movements, a conventional “Entrance-Exit” route setting control panel is probably the most “user-friendly” and efficient method of route setting. IECC and similar highly automated systems, based on timetabled operation are most suited for mainline/metro operation, where little shunting and non-timetabled operations take place. With the introduction of IECC operator workload was further reduced with such features as Automatic Route Setting (ARS) and Train Operated Route Release. (TORR). (However it should be noted that TORR had previously been incorporated into the previous generation of relay interlocked systems, operated from conventional push button control panels. ARS previously had been adopted on a limited trial on the Three Bridges resignalling scheme, commissioned in 1983). 21st century Computer Based Interlockings (CBI) are produced by a variety of manufacturers and have proven to be reliable and safe. CBI/SSI systems are now used worldwide for most new signalling installations. However, in terms of evolution, CBI systems have yet to meet their full potential. Whilst there is no doubt as to the reliability and safety of SSI and CBI systems, it is interesting to note that all such systems presently on the market are slower than an all relay based interlocking. This isn’t usually a problem. However certain timing problems can occur where a signalled route crosses the boundary of two interlocking areas. These problems that were identified during the early days of SSI and CBI systems have now been resolved.

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Notable exceptions to introduction of CBI are when an existing line operated by relay based interlockings is being extended by a relatively small distance. In this case it can make sense to equip the new line with relay interlockings. This reduces risk by eliminating application development for a CBI system to adapt it to a relatively small installation. In addition, potential interface problems are avoided. Examples of recent new lines in Singapore that have/will be equipped with relay based interlockings are Changi Airport line (opened in 2001) and Boon Lay Extension (Due to open in 2008). These both interface to existing relay based systems. All other new lines in future will utilise Computer Based Interlockings. In addition, with the latest technology, solid state switching of ATP codes is possible, eliminating the need for relay based switching of ATP codes. In the UK, examples of schemes recently carried out using relay interlockings instead of CBI were at Longsight, South of Manchester. In this case relays that had been in operation since early 1960s were replaced with new relays, rather than convert to SSI. In this particular case, relay interlockings were maintained to reduce work associated with interfacing to existing control centre and trackside equipment that are relatively new. Further South at Stockport, several mechanical signal boxes some dating from 1880s have been refurbished, and interfacing circuitry rewired to new BR930 style relays. The decision to refurbish the existing signal boxes using a mixture of mechanical and relay technology at Stockport was taken after delays in the development/delivery of a European CBI system. Such cases are the exception, but it shows that in particular cases, there is still a demand for traditional technology. It should be noted that whilst the interlocking function is no longer carried out by relays in Computer based interlockings, there remains a requirement for relays to perform an interfacing role. This can be for example between the CBI and trackside equipment such as point machines and track circuits. (Note that in the CBI system used on NEL and CCL in Singapore, the track circuit receivers interface directly with the interlocking, without the use of interface Track relays) Relay interfaces also provide a very good electrical isolation between the delicate electronics of a Computer Based System and the harsh external environment.

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However, it should be noted that systems such as Solid State Interlocking developed in UK, were designed for a minimum requirement for interfacing relays. In SSI, specific modules drive signals and points directly without interface relays. However this system was primarily designed for the UK market. (Though it was later adapted for use elsewhere). Most other CBI systems developed for world wide application, tend to use more interface relays. This enables one design of module to be used to drive a wide range of different types of trackside equipment. Examples of extensive use of relays for interface purposes in conjunction with Computer Based Interlockings can be seen on North East Line and Circle Line in Singapore. On these systems, cabinets housing a range of FS90 style relays are provided to interface to the CBI system. These relays typically operate at 24V DC. They are made in Bologna, Italy by Alstom at the former SASIB factory. Relays of similar design are used extensively in Italy in large relay interlockings (Milan Central being a good example). These relays continue to be produced and are used in the latest CBI installations in Italy and other countries in Europe, notably the Netherlands. In Singapore on NEL and CCL, the ASCV interlocking does interface to trackside signals and track circuits, without the use of interface relays. However circuits such as ESP (Emergency Stop Plunger), SPKS (Staff Protection Keyswitch circuits, do use a relay interface on NEL and CCL. BPLRT in Singapore uses a CBI in conjunction with interface relays manufactured by the Union Switch and Signal Company in the United States. These are typically used for point interface purposes and are housed in cabinets adjacent to the guideway. These are plug in type relays, slightly larger than the UK 930 series relays. The Downtown Line currently under construction in Singapore, will make extensive use of Westinghouse Q type relays for interface purposes. These will interface to the Westrace Mk 2 CBI. Similar relays of the PN150 series are used extensively in the United States and Canada. In the US and Canada, signalling relays typically operate at 10 or 12 Volts DC. Interestingly, some railroads in the United States were relatively slow to make the transition from “Fish Tank” to plug in relays. In Malaysia, north of Kuala Lumpur, the line from Rwang to Ipoh is currently being re-signalled, in conjunction with track doubling and electrification. Microlok Interlocking, is being supplied by Union Switch and Signal (Australia).

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Use of interface relays enables isolation of the interlocking from trackside equipment. In this case 24V relays to BR930 specification are supplied by Crompton Greaves of India. It should be noted however, that these relays don’t conform to the latest UK Network Rail Specifications. In the UK on the projects to resignal London Underground Victoria Line and Sub Surface Lines (SSL), the latest generation of Westrace CBI is used for interlocking. These are interfaced to trackside and station equipment by 50V Q style relays to BR 930 spec, manufactured by Westinghouse Rail systems. In addition on the Victoria Line project, a temporary relay interface is used to provide the “Overlay phase” in which the original signalling system supplied in the mid 1960s is interfaced to the latest technology signalling systems being supplied as part of the PPP upgrade project. In this case the relay interface provides a vital and flexible interface to enable two very different signalling systems operating at different system voltages and frequencies to “talk” to each other. The relay interface ensures complete electrical isolation between the two systems. Conclusion Signalling interlocking has evolved in the following main phases:

• Mechanical interlockings first introduced around the mid 19th century, probably reached the peak development in the mid 1930s, after the introduction of such features as interlinking of signal and point controls with track circuits and block controls. (However they continued to be widely used on mainlines in UK well into the 1970/1980s. There are still examples of mechanical signalling boxes on UK mainlines, but these are generally isolated examples, operating colour light signals via electric circuit controllers. Mechanical signal boxes operating mechanical signals can still be found on secondary lines and goods only lines).

• Interlockings operated from miniature lever frames replicated directly

mechanical interlockings with full size levers. Interlocking between the miniature levers was initially mechanical as well, but this first changed to electrical interlocking in 1929. Production of these continued well into the 1950s.

• Relay interlockings first introduced in the 1930s were a logical

development from the interlockings operated from miniature lever frames that had been introduced in the 1920s. Relay interlockings are still widely used in installations dating from 1960/-/1980s. Relay interlockings combined with “Entrance-Exit” push button control and train describers, significantly reduced operator workload.

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• Solid State Interlockings (SSI) and Computer Based Interlockings (CBI)

were first introduced in 1980s. From the operator’s point of view, these are similar to relay based systems. They are now widely used for most new signalling schemes worldwide. At the transition from one main type of signalling to the next generation, a number of “hybrid” schemes of relatively limited application were introduced. Even in Singapore, examples of all main types of signalling system described above can be found.

• On the Malaysian Railway (KTMB) mechanical interlockings dating from 1920/1930s can be found e.g. at Bukit Timah.

• Relay interlockings are universally used on NS-EW SMRT lines. These

were first introduced in 1980s, based on the UK BR interlockings of the time, but adopted to a metro application.

• CBI is used exclusively on NEL, BPLRT and Sengkang/Punggol LRT.

In the UK in 2003, it was estimated that there were a total of 1,700 interlockings in existence.13 The breakdown was as follows Interlocking type Quantity (Approx) Estimated average age Mechanical 600 75 Relay based 850 25 Computer based 250 7 It will be seen that in UK, the most common type of interlocking is still relay based. These probably overtook mechanical interlockings, as being the most common type around late 1970s. However, it is interesting to note that despite introduction of major re-signalling schemes in UK over the last 40 years, there are still a large number of mechanical interlockings in existence. At the start of the 21st century, the number of Computer based interlockings still remain the lowest of all types. A great many relay based interlockings (particularly those commissioned in the 1960s to 1980s) are now reaching the end of their lives. However due to the large number of relay based schemes in operation, it will probably be many years until the majority of interlockings in UK are computer based. This is despite the fact that most new signalling schemes in UK now use computer based interlockings. This is largely as a result of an investment backlog that has built up in recent years. In another example, on the Pro Rail Network in Netherlands, it is estimated that in March 2007, there were 300 interlockings in existence.

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Of these, 230 were relay based and 70 were “electronic”. (Presumably Computer Based Interlockings).14 i.e as at 2007, CBI interlockings in Netherlands are estimated to be only 23% of the total and relay based interlockings still make up 76% of the total. The introduction of each new phase has brought about reductions in the initial cost of the signalling systems, and benefits for the operators mainly in terms of reduced workload and increased productivity. Balanced against this, the life span of the new signalling systems are less than the previous relay systems. The relay based systems in turn have a shorter lifespan than the previous generation of mechanical systems. The real long term costs of adopting the latest technology remain to be seen. The greatest step forward from an operating viewpoint, was probably the transition from mechanical to relay based systems, particularly when combined with the introduction of track circuit block and train describers that eliminated the need for operation of manual block. The change from relay based to SSI/CBI systems, whilst a great technological leap, was almost invisible to the operator, as the new systems basically emulate a relay based system. Indeed many of the operational features of the latest SSI/CBI interlockings, can be traced back to the early relay interlockings introduced in the 1930s. Production of signalling relays peaked in the UK in the late 1970s/early 1980s, coinciding with the last large relay based signalling schemes. Since then the widespread adoption of Solid State and Computer based interlockings has steadily reduced the demand for signalling relays, however it is interesting to note that a new BR930 relay was developed as an interface relay to SSI. This operates at 110V AC. Relay production has now stabilised, but thousands of relays continue to be manufactured each year. There are now only two major suppliers of signalling relays in UK. These are Invensys (formerly Westinghouse Rail Systems of Chippenham, Wiltshire (www.westsig.co.uk) and STS-Signals Limited of Cradley Heath, West Midlands. (www.sts-signals.com). Invensys Rail systems reported their highest ever production of over 2,000 signalling relays in one week in 2010. This shows that there is still a high demand for signaling relays, despite most new signaling schemes using SSI or CBI systems for interlocking. STS –Signals manufacture relays to the same designs that were originally manufactured by Tyers, Field and Grant and GEC General Signal Limited. (Later Alstom UK).

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In the UK, relays can also be purchased from Unipart Rail. (www.unipartrail.com). This company supplies new and reconditioned relays, manufactured by Invensys Rail Systems and STS-Signals and it’s predecessor companies. Relays will be required for the foreseeable future, mainly for interfacing purposes on SSI/CBI systems and where short extensions of existing relay based systems are being carried out. In addition, there is a steady demand for supply of relays for replacement of life expired relays in existing relay based systems. From time to time, owing to particular circumstances, relay interlockings are retained and upgraded with relay based systems, rather than SSI/CBI. In Singapore, the existing NS-EW line continues to use relays for interlocking and ATP code switching. However, it should be noted that trials are currently underway at Bishan Depot to confirm feasibility of switching ATP codes by Computer Based Interlocking. All future schemes in Singapore will use CBI with interface relays where appropriate. References 1 Red for Danger. L.T.C. Rolt, 1966 reprint 2 IRSE students prize essay “The uses of electricity in signalling”. Leonard C. Lang 1932 3 IRSE Paper “Railway Colour Light Signalling in relation to manual block and Multiple Aspect Signals. A.F. Bound 1932 4 The Style L Power Frame. J.D. Francis 1989 5 A Hundred Years of Speed with Safety. O.S.Nock 1981. Published 2006, edited by Stuart Angill, John Francis, Mark Glover, Michael Stone. Published by The Hobnob Press. 6 IRSE Presidential Address. T.S. Howard 1988 7 Signalling in the Age of Steam. Michael A. Vanns 1995 8 IRSE paper “ All Electric Interlocking frames versus Relay Interlocking Control Panels. C.F.D Venning 1949 9 IRSE Paper “Design for Signalling System Performance.” P.W. Stanley 1980 10 General Information on Style Q relays. Westinghouse Rail Systems data sheet. 11 Article in IET Computing and Control Magazine titled “Signalling Technology for today’s railway.” Mark Glover 2007. 12 IRSE Paper “A comparison between relay and electric lever locking”. E.W. Challis 1938 13 IRSE Paper “Sustainable Interlocking for the 21st Century”. Kenneth Vine and Philip Hingley 2003 14 IRSE Paper “Dutch Signalling Developments” Maarten van der Werff 2007 Note : Full details (in the form of formal accident reports) for most of the railway accidents referred to in this article can be accessed at www.railwaysarchive.co.uk IRSE ARTICLE Signalling relays.doc MPW 9 June 2010