Inrailway signalling, aninterlocking is an arrangement of signal apparatus that prevents conflicting movements through an arrangement of tracks such asjunctions or crossings. In North America, a set of signalling appliances andtracks interlocked together are sometimes collectively referred to as aninterlocking plant or just as aninterlocking. An interlocking system is designed so that it is impossible to display a signal to proceed unless the route to be used is proven safe.
Interlocking is a safety measure designed to prevent signals andpoints/switches from being changed in an improper sequence. For example, interlocking would prevent a signal from being changed to indicate a diverging route, unless the corresponding points/switches had been changed first. In North America, the official railroad definition of interlocking is: "An arrangement of signals and signal appliances so interconnected that their movements must succeed each other in proper sequence".[1]
A minimal interlocking consists ofsignals, but usually includes additional appliances such aspoints and Facing Point locks (US: switches) andderails, and may include crossings at grade and movable bridges. Some of the fundamental principles of interlocking include:
Railway interlocking is ofBritish origin, where numerous patents were granted. In June 1856,John Saxby received the first patent for interlocking switches and signals.[2][3]: 23–24 In 1868, Saxby (of Saxby & Farmer)[4] was awarded a patent for what is known today in North America as “preliminary latch locking”.[5][6] Preliminary latch locking became so successful that by 1873, 13,000 mechanical locking levers were employed on theLondon and North Western Railway alone.[6][7]
The first experiment with mechanical interlocking in the United States took place in 1875 by J. M. Toucey and William Buchanan atSpuyten Duyvil Junction inNew York on theNew York Central and Hudson River Railroad (NYC&HRR).[6][7][8] At the time, Toucey was General Superintendent and Buchanan was Superintendent of Machinery on the NYC&HRR. Toucey and Buchanan formed the Toucey and Buchanan Interlocking Switch and Signal Company inHarrisburg, Pennsylvania in 1878. The first important installations of their mechanism were on the switches and signals of theManhattan Elevated Railroad Company and the New York Elevated Railroad Company in 1877–78.[6] Compared to Saxby's design, Toucey and Buchanans' interlocking mechanism was more cumbersome and less sophisticated, and so was not implemented very widely.[8]Union Switch & Signal bought their company in 1882.[8]
As technology advanced the railway signaling industry looked to incorporate these new technologies into interlockings to increase the speed of route setting, the number of appliances controlled from a single point and to expand the distance that those same appliances could be operated from the point of control. The challenge facing the signal industry was achieving the same level of safety and reliability that was inherent to purely mechanical systems. An experimentalhydro-pneumatic[9] interlocking was installed at theBound Brook, New Jersey junction of thePhiladelphia and Reading Railroad and theLehigh Valley Railroad in 1884.[6][7] By 1891, there were 18 hydro-pneumatic plants, on six railroads, operating a total of 482 levers.[6] The installations worked, but there were serious defects in the design, and little saving of labour was achieved.
The inventors of the hydro-pneumatic system moved forward to an electro-pneumatic system in 1891 and this system, best identified with the Union Switch & Signal Company, was first installed on theChicago and Northern Pacific Railroad at its drawbridge across theChicago River.[7] By 1900, 54 electro-pneumatic interlocking plants, controlling a total of 1,864 interlocking levers, were in use on 13 North American railroads. This type of system would remain one of two viable competing systems into the future, although it did have the disadvantage of needing extra single-use equipment and requiring high maintenance.[7]
Interlockings using electric motors for moving switches and signals became viable in 1894, whenSiemens in Austria installed the first such interlocking atPřerov (now in the Czech Republic).[10] Another interlocking of this type was installed inWestend nearBerlin in 1896.[11] In North America, the first installation of an interlocking plant using electric switch machines was atEau Claire, Wisconsin on theChicago, St. Paul, Minneapolis and Omaha Railway in 1901, byGeneral Railway Signal Company (GRS, now a unit ofAlstom, headquartered inLevallois-Perret, near Paris).[7] By 1913, this type system had been installed on 83 railroads across 35 US states and Canadian provinces, in 440 interlocking plants using 21,370 levers.[6]
Interlockings can be categorized as mechanical, electrical (electro-mechanical orrelay-based), or electronic/computer-based.
In mechanical interlocking plants, alocking bed is constructed, consisting of steel bars forming a grid. Thelevers that operateswitches,derails, signals or other appliances are connected to the bars running in one direction. The bars are constructed so that if the function controlled by a given lever conflicts with that controlled by another lever, mechanical interference is set up in thecross locking between the two bars, in turn preventing the conflicting lever movement from being made.
In purely mechanical plants, the levers operate the field devices, such as signals, directly via a mechanical rodding or wire connection. The levers are about shoulder height since they must supply a mechanical advantage for the operator. Cross locking of levers was effected such that the extra leverage could not defeat the locking (preliminary latch lock).
The first mechanical interlocking was installed in 1843 atBricklayers Arms Junction, England.[12]: 7
Power interlockings may also use mechanical locking to ensure the proper sequencing of levers, but the levers are considerably smaller as they themselves do not directly control the field devices. If the lever is free to move based on the locking bed, contacts on the levers actuate the switches and signals which are operated electrically or electro-pneumatically. Before a control lever may be moved into a position which would release other levers, a signal must be received from the field element that it has actually moved into the position requested. The locking bed shown is for a GRS power interlocking machine.
Interlockings effected purely electrically (sometimes referred to asall-electric) consist of complex circuitry made up ofrelays in an arrangement ofrelay logic that ascertain the state or position of each signal appliance. As appliances are operated, their change of position opens some circuits that lock out other appliances that would conflict with the new position. Similarly, other circuits are closed when the appliances they control become safe to operate. Equipment used for railroad signalling tends to be expensive because of its specialized nature andfail-safe design.
Interlockings operated solely by electrical circuitry may be operated locally or remotely, with the large mechanical levers of previous systems being replaced by buttons, switches or toggles on a panel or video interface. Such an interlocking may also be designed to operate without a human operator. These arrangements are termedautomatic interlockings, and the approach of a train sets its own route automatically, provided no conflicting movements are in progress.
GRS manufactured the first all-relay interlocking system in 1929. It was installed inLincoln, Nebraska on theChicago, Burlington and Quincy Railroad.[12]: 18
Entrance-Exit Interlocking (NX) was the original brand name of the first generation relay-basedcentralized traffic control (CTC) interlocking system introduced in 1936 by GRS[13] (represented in Europe byMetropolitan-Vickers). The advent of all electric interlocking technology allowed for more automated route setting procedures as opposed to having an operator line each part of the route manually. The NX system allowed an operator looking at the diagram of a complicated junction to simply push a button on the known entrance track and another button on the desired exit track. The logic circuitry handled all the necessary actions of commanding the underlying relay interlocking to set signals and throw switches in the proper sequence, as required to provide valid route through the interlocking plant. The first NX installation was in 1937 atBrunswick on theCheshire Lines, UK. The first US installation was on theNew York Central Railroad (NYCRR) at Girard Junction, Ohio in 1937.[12]: 18 Another NYCRR installation was on the main line betweenUtica, New York andRochester, New York, and this was quickly followed up by three installations on theNew York City Subway'sIND Fulton Street Line in 1948.[14][15]
Other NX style systems were implemented by other railroad signal providers. For example,Union Route (UR) was the brand name of their Entrance-Exit system supplied byUnion Switch & Signal Co. (US&S), and introduced in 1951.[16] NX type systems and their costly pre-solid state control logic only tended to be installed in the busier or more complicated terminal areas where it could increase capacity and reduce staffing requirements. In a move that was popular in Europe, the signalling for an entire area was condensed into a single largepower signal box with a control panel in the operator's area and the equivalent of atelephone exchange in the floors below that combined the vital relay based interlocking logic and non-vital control logic in one place. Such advanced schemes would also includetrain describer and train tracking technologies. Away from complex terminals unit lever control systems remained popular until the 1980s when solid state interlocking and control systems began to replace the older relay plants of all types.
Modern interlockings (those installed since the late 1980s) are generallysolid state, where the wired networks of relays are replaced by software logic running on special-purpose control hardware.[3]: 84 The fact that the logic is implemented by software rather than hard-wired circuitry greatly facilitates the ability to make modifications when needed by reprogramming rather than rewiring. In many implementations, thisvital logic is stored asfirmware or inROM that cannot be easily altered to both resist unsafe modification and meet regulatory safety testing requirements. As display technology improved, the hard wired physical devices could be updated withvisual display units (computer monitors), which allowed changes in field equipment be represented to the signaller without any hardware modifications.
"Solid State Interlocking" (SSI) is the brand name in trade of work of the first generationmicroprocessor-based interlocking developed in the 1980s byBritish Rail, GEC-General Signal andWestinghouse Signals Ltd in the UK. Second generation processor-based interlockings are known by the term "Computer Based Interlocking" (CBI),[17] of whichVPI (trademark ofGeneral Railway Signal, now Alstom),MicroLok (trademark ofUnion Switch & Signal, nowHitachi Rail STS),Westlock andWestrace (trademarks ofInvensys Rail, now Siemens),Smartlock[18] (trademark ofAlstom), andEBI Lock (trademark ofBombardier) are examples.
Interlockings allow trains to cross from one track to another using aturnout and a series of switches. Railroad terminology defines the following types of interlockings as eithercomplete orincomplete depending on the movements available. Although timetables generally do not identify an interlocking as one or the other, and rule books do not define the terms, the terms below are generally agreed upon by system crews and rules officials.
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