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railway signalling Railway signalling (), also called railroad signaling (), is a system used to control the movement of railway traffic. Trains move on fixed rails, making them uniquely susceptible to collision. This susceptibility is exacerbated by the enormo ...
, an interlocking is an arrangement of signal apparatus that prevents conflicting movements through an arrangement of tracks such as
junction Junction may refer to: Arts and entertainment * ''Junction'' (film), a 2012 American film * Jjunction, a 2002 Indian film * Junction (album), a 1976 album by Andrew Cyrille * Junction (EP), by Basement Jaxx, 2002 * Junction (manga), or ''Hot ...
s or crossings. The signalling appliances and tracks are sometimes collectively referred to as an ''interlocking plant''. An interlocking 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 and points/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''".


Configuration and use

A minimal interlocking consists of signals, but usually includes additional appliances such as
points Point or points may refer to: Places * Point, Lewis, a peninsula in the Outer Hebrides, Scotland * Point, Texas, a city in Rains County, Texas, United States * Point, the NE tip and a ferry terminal of Lismore, Inner Hebrides, Scotland * Points ...
and Facing Point locks (US: switches) and derails, and may include crossings at grade and movable bridges. Some of the fundamental principles of interlocking include: * Signals may not be operated to permit conflicting train movements to take place at the same time on set route. * Switches and other appliances in the route must be properly 'set' (in position) before a signal may allow train movements to enter that route. * Once a route is ''set'' and a train is given a signal to proceed over that route, all switches and other movable appliances in the route are locked in position until either ** the train passes out of the portion of the route affected, or ** the signal to proceed is withdrawn and sufficient time has passed to ensure that a train approaching that route has had opportunity to come to a stop before passing the signal.


History

Railway interlocking is of British origin, where numerous patents were granted. In June 1856, John Saxby received the first patent for interlocking switches and signals. In 1868, Saxby (of Saxby & Farmer) was awarded a patent for what is known today in North America as “preliminary latch locking”. Preliminary latch locking became so successful that by 1873, 13,000 mechanical locking levers were employed on the
London and North Western Railway The London and North Western Railway (LNWR, L&NWR) was a British railway company between 1846 and 1922. In the late 19th century, the L&NWR was the largest joint stock company in the United Kingdom. In 1923, it became a constituent of the Lo ...
alone. The first experiment with mechanical interlocking in the United States took place in 1875 by J. M. Toucey and William Buchanan at Spuyten Duyvil Junction in New York on the
New York Central and Hudson River Railroad The New York Central Railroad was a railroad primarily operating in the Great Lakes and Mid-Atlantic regions of the United States. The railroad primarily connected greater New York and Boston in the east with Chicago and St. Louis in the Mid ...
(NYC&HRR). 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 in
Harrisburg, Pennsylvania Harrisburg is the capital city of the Commonwealth of Pennsylvania, United States, and the county seat of Dauphin County. With a population of 50,135 as of the 2021 census, Harrisburg is the 9th largest city and 15th largest municipality in ...
in 1878. The first important installations of their mechanism were on the switches and signals of the Manhattan Elevated Railroad Company and the New York Elevated Railroad Company in 1877–78. Compared to Saxby's design, Toucey and Buchanans' interlocking mechanism was more cumbersome and less sophisticated, and so was not implemented very widely. Union Switch & Signal bought their company in 1882. 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 experimental hydro-pneumatic interlocking was installed at the
Bound Brook, New Jersey Bound Brook is a borough in Somerset County, New Jersey, United States, located along the Raritan River. At the 2010 United States Census, the borough's population was 10,402,Philadelphia and Reading Railroad and the
Lehigh Valley Railroad The Lehigh Valley Railroad was a railroad built in the Northeastern United States to haul anthracite coal from the Coal Region in Pennsylvania. The railroad was authorized on April 21, 1846 for freight and transportation of passengers, goods, ...
in 1884. By 1891, there were 18 hydro-pneumatic plants, on six railroads, operating a total of 482 levers. 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 the
Chicago and Northern Pacific Railroad (''City in a Garden''); I Will , image_map = , map_caption = Interactive Map of Chicago , coordinates = , coordinates_footnotes = , subdivision_type = Country , subdivision_name ...
at its drawbridge across the
Chicago River The Chicago River is a system of rivers and canals with a combined length of that runs through the city of Chicago, including its center (the Chicago Loop). Though not especially long, the river is notable because it is one of the reasons fo ...
. 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. Interlockings using electric motors for moving switches and signals became viable in 1894, when
Siemens Siemens AG ( ) is a German multinational conglomerate corporation and the largest industrial manufacturing company in Europe headquartered in Munich with branch offices abroad. The principal divisions of the corporation are ''Industry'', ''E ...
in Austria installed the first such interlocking at
Přerov Přerov (; german: Prerau) is a city in the Olomouc Region of the Czech Republic. It has about 41,000 inhabitants. It lies on the Bečva River. In the past it was a major crossroad in the heart of Moravia in the Czech Republic. The historic cent ...
(now in the Czech Republic).
Lexikon der gesamten Technik The ''Lexikon der gesamten Technik'' is an illustrated German-language encyclopedia of architectural, engineering and manufacturing technology, written by Otto Lueger (German engineer, 1843–1911) and first published in 1894. Editions ...
, entry "Stellwerke"
Another interlocking of this type was installed in
Westend Westend may refer to: * Westend (Trevilians, Virginia), an historic house in Virginia listed on the NRHP * Westend (Berlin), a locality of Berlin in Germany * Westend (Frankfurt am Main), a borough of Frankfurt am Main in Germany * Westend, Espoo, ...
near
Berlin Berlin ( , ) is the capital and largest city of Germany by both area and population. Its 3.7 million inhabitants make it the European Union's most populous city, according to population within city limits. One of Germany's sixteen constitu ...
in 1896. In North America, the first installation of an interlocking plant using electric switch machines was at Eau Claire, Wisconsin on the
Chicago, St. Paul, Minneapolis and Omaha Railway The Chicago, St. Paul, Minneapolis and Omaha Railway or ''Omaha Road'' was a railroad in the U.S. states of Nebraska, Iowa, Minnesota, Wisconsin and South Dakota. It was incorporated in 1880 as a consolidation of the Chicago, St. Paul and Minne ...
in 1901, by General Railway Signal Company (GRS, now a unit of
Alstom Alstom SA is a French multinational corporation, multinational rolling stock manufacturer operating worldwide in rail transport markets, active in the fields of passenger transportation, signalling, and locomotives, with products including the A ...
, headquartered in Levallois-Perret, near Paris). By 1913, this type system had been installed on 83 railroads in 35 US States and Canadian Provinces, in 440 interlocking plants using 21,370 levers.


Interlocking types

Interlockings can be categorized as mechanical, electrical (electro-mechanical or
relay A relay Electromechanical relay schematic showing a control coil, four pairs of normally open and one pair of normally closed contacts An automotive-style miniature relay with the dust cover taken off A relay is an electrically operated switch ...
-based), or electronic/ computer-based.


Mechanical interlocking

In mechanical interlocking plants, a ''locking bed'' is constructed, consisting of steel bars forming a grid. The
lever A lever is a simple machine consisting of a beam or rigid rod pivoted at a fixed hinge, or '' fulcrum''. A lever is a rigid body capable of rotating on a point on itself. On the basis of the locations of fulcrum, load and effort, the lever is d ...
s that operate switches, 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 the ''cross 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 at Bricklayers Arms Junction, England.


Electro-mechanical interlocking

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.


Relay interlocking

Interlockings effected purely electrically (sometimes referred to as "''all-electric''") consist of complex circuitry made up of
relay A relay Electromechanical relay schematic showing a control coil, four pairs of normally open and one pair of normally closed contacts An automotive-style miniature relay with the dust cover taken off A relay is an electrically operated switch ...
s in an arrangement of relay 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 and fail-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 termed ''automatic 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 in
Lincoln, Nebraska Lincoln is the capital city of the U.S. state of Nebraska and the county seat of Lancaster County. The city covers with a population of 292,657 in 2021. It is the second-most populous city in Nebraska and the 73rd-largest in the United ...
on the
Chicago, Burlington and Quincy Railroad The Chicago, Burlington and Quincy Railroad was a railroad that operated in the Midwestern United States. Commonly referred to as the Burlington Route, the Burlington, or as the Q, it operated extensive trackage in the states of Colorado, Illin ...
. Entrance-Exit Interlocking (NX) was the original brand name of the first generation relay-based
centralized traffic control Centralized traffic control (CTC) is a form of railway signalling that originated in North America. CTC consolidates train routing decisions that were previously carried out by local signal operators or the train crews themselves. The system con ...
(CTC) interlocking system introduced in 1936 by GRS (represented in Europe by Metropolitan-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 at Brunswick on the Cheshire Lines, UK. The first US installation was on the
New York Central Railroad The New York Central Railroad was a railroad primarily operating in the Great Lakes and Mid-Atlantic regions of the United States. The railroad primarily connected greater New York and Boston in the east with Chicago and St. Louis in the Mi ...
(NYCRR) at Girard Junction, Ohio in 1937. Another NYCRR installation was on the main line between
Utica, New York Utica () is a city in the Mohawk Valley and the county seat of Oneida County, New York, United States. The tenth-most-populous city in New York State, its population was 65,283 in the 2020 U.S. Census. Located on the Mohawk River at the fo ...
and
Rochester, New York Rochester () is a City (New York), city in the U.S. state of New York (state), New York, the county seat, seat of Monroe County, New York, Monroe County, and the fourth-most populous in the state after New York City, Buffalo, New York, Buffalo, ...
, and this was quickly followed up by three installations on the
New York City Subway The New York City Subway is a rapid transit system owned by the government of New York City and leased to the New York City Transit Authority, an affiliate agency of the state-run Metropolitan Transportation Authority (MTA). Opened on October ...
's
IND Fulton Street Line The IND Fulton Street Line is a rapid transit line of the IND Division of the New York City Subway, running from the Cranberry Street Tunnel under the East River through all of central Brooklyn to a terminus in Ozone Park, Queens. The IND Ro ...
in 1948. 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 by Union Switch & Signal Co. (US&S), and introduced in 1951. 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 large power signal box with a control panel in the operator's area and the equivalent of a telephone 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 include train 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.


Electronic interlocking

Modern interlockings (those installed since the late 1980s) are generally solid state, where the wired networks of relays are replaced by software logic running on special-purpose control hardware. 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, this vital logic is stored as
firmware In computing, firmware is a specific class of computer software that provides the low-level control for a device's specific hardware. Firmware, such as the BIOS of a personal computer, may contain basic functions of a device, and may provide h ...
or in ROM that cannot be easily altered to both resist unsafe modification and meet regulatory safety testing requirements. At this time there were also changes in the systems that controlled interlockings. Whereas before technologies such as NX and Automatic Route Setting required racks and racks of relays and other devices, solid state software based systems could handle such functions with less cost and physical footprint. Initially processor driven Unit Lever and NX panels could be set up to command field equipment of either electronic or relay type; however as display technology improved, these hard wired physical devices could be updated with visual display units, which allowed changes in field equipment be represented to the signaller without any hardware modifications.
Solid State Interlocking Solid is one of the four fundamental states of matter (the others being liquid, gas, and plasma). The molecules in a solid are closely packed together and contain the least amount of kinetic energy. A solid is characterized by structural ...
(SSI) is the brand name in trade of work of the first generation
microprocessor A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circ ...
-based interlocking developed in the 1980s by
British Rail British Railways (BR), which from 1965 traded as British Rail, was a state-owned company that operated most of the overground rail transport in Great Britain from 1948 to 1997. It was formed from the nationalisation of the Big Four (British ra ...
, GEC-General Signal and Westinghouse Signals Ltd in the UK. Second generation processor-based interlockings are known by the term "Computer Based Interlocking" (CBI), of which VPI (trademark of General Railway Signal, now Alstom), MicroLok (trademark of Union Switch & Signal, now Hitachi Rail STS), Westlock and Westrace (trademarks of Invensys Rail, now Siemens), and martlock(trademark of
Alstom Alstom SA is a French multinational corporation, multinational rolling stock manufacturer operating worldwide in rail transport markets, active in the fields of passenger transportation, signalling, and locomotives, with products including the A ...
), and EBI Lock (trademark of Bombardier) are examples.


Defined forms of locking

; Electric locking : "The combination of one or more electric locks or controlling circuits by means of which levers in an interlocking machine, or switches or other devices operated in connection with signalling and interlocking, are secured against operation under certain conditions."Defined by the Railway Signal Association, which today is the Railway Signal Committee of the Association of American Railroads. ; Section locking : "Electric locking effective while a train occupies a given section of a route and adapted to prevent manipulation of levers that would endanger the train while it is within that section." ; Route locking : "Electric locking taking effect when a train passes a signal and adapted to prevent manipulation of levers that would endanger the train while it is within the limits of the route entered." ; Sectional route locking : "Route locking so arranged that a train, in clearing each section of the route, releases the locking affecting that section." ; Approach locking : "Electric locking effective while a train is approaching a signal that has been set for it to proceed and adapted to prevent manipulation of levers or devices that would endanger that train." ; Stick locking : "Electric locking taking effect upon the setting of a signal for a train to proceed, released by a passing train, and adapted to prevent manipulation of levers that would endanger an approaching train." ; Indication locking : "Electric locking adapted to prevent any manipulation of levers that would bring about an unsafe condition in case a signal, switch, or other operated device fails to make a movement corresponding with that of the operating lever; or adapted directly to prevent the operation of one device in case another device, to be operated first, fails to make the required movement." ; Check locking or traffic locking : "Electric locking that enforces cooperation between the Operators at two adjacent plants in such a manner that prevents opposing signals governing the same track from being set to proceed at the same time. In addition, after a signal has been cleared and accepted by a train, check locking prevents an opposing signal at the adjacent interlocking plant from being cleared until the train has passed through that plant."


Complete and incomplete interlockings (U.S. terminology)

Interlockings allow trains to cross from one track to another using a turnout and a series of switches. Railroad terminology defines the following types of interlockings as either ''complete'' or ''incomplete'' 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. ; Complete interlockings : allow continuous movements from any track on one side of the interlocking to any track on the opposite side without the use of a reverse move within the limits of the interlocking. This is true even if there are differing numbers of tracks on opposing sides, or if the interlocking has multiple sides. ; Incomplete interlockings : do not allow such movements as described above. Movements in an incomplete interlocking may be limited and may even require reverse movements to achieve the desired route.


See also

* Breath alcohol ignition interlock device * Fail-safe *
Interlock (Engineering) An interlock is a feature that makes the state of two mechanisms or functions mutually dependent. It may be used to prevent undesired states in a finite-state machine, and may consist of any electrical, electronic, or mechanical devices or system ...
* Lockout-tagout * Safety instrumented system *
Signalling control On a rail transport system, signalling control is the process by which control is exercised over train movements by way of railway signals and block systems to ensure that trains operate safely, over the correct route and to the proper timetab ...


References

* * Ganguly, Sri Subhasis
"History of Railway Signalling."
Accessed 2011-05-06. *


External links

* Calvert, J.B



* ttp://www.signalsimulation.com/downloads/Kleinstadt.zip Kleinstadt.zip "Full free version of an interlocking plant based on German Relay Principles" (English, German, Dutch, French languages)* Director of S&T Engineering, West Midlands Project Grou
"Mechanical Interlocking."
{{Rail tracks Interlocking systems Rail infrastructure Railway signalling control es:Enclavamiento sv:Ställverk (järnväg)