Editing Interlocking (section)

== Interlocking types ==
Interlockings can be categorized as mechanical, electrical (electro-mechanical or [[relay]]-based), or electronic/[[Computer-based interlocking|computer-based]].

=== Mechanical interlocking ===
{{See also|Lever frame}}
[[File:Interlocking machine locking bed.jpg|thumb|upright|A view of the locking bed inside Deval Tower, [[Des Plaines, Illinois]]]]

In mechanical interlocking plants, a ''locking bed'' is constructed, consisting of steel bars forming a grid. The [[lever]]s that operate [[Railroad switch|switches]], [[Derail (railroad)|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 railway station|Bricklayers Arms Junction]], England.<ref name="Alstom History">{{cite book|title=A Centennial: History of Alstom Signaling Inc.|author=Alstom Signaling Incorporated|date=2004|publisher=Alstom|location=West Henrietta, New York|url=http://www.alstomsignalingsolutions.com/Data/Documents/History.pdf|access-date=27 December 2011|url-status=dead|archive-url=https://web.archive.org/web/20111002122301/http://www.alstomsignalingsolutions.com/Data/Documents/History.pdf|archive-date=2 October 2011}}</ref>{{rp|7}}

=== 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-[[pneumatics|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 ===
[[File:Relay room.jpg|thumb|Part of a relay interlocking using miniature plug-in relays]]

Interlockings effected purely electrically (sometimes referred to as ''all-electric'') consist of complex circuitry made up of [[relay]]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]] on the [[Chicago, Burlington and Quincy Railroad]].<ref name="Alstom History" />{{rp|18}}

[[File:Promenade St Tower Control Panel.jpg|thumb|Control panel for a US&S relay interlocking]]

'''Entrance-Exit Interlocking (NX)''' was the original brand name of the first generation relay-based [[centralized traffic control]] (CTC) interlocking system introduced in 1936 by GRS<ref>{{cite book|title=The NX System of Electric Interlocking|author=General Railway Signal Company|date=1936|location=Rochester, New York|url=http://www.rrsignalpix.com/GRS_NX.pdf|oclc=184909207|url-status=live|archive-url=https://web.archive.org/web/20101128220310/http://rrsignalpix.com/GRS_NX.pdf|archive-date=2010-11-28}}</ref> (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 railway station|Brunswick]] on the [[Cheshire Lines Committee|Cheshire Lines]], UK. The first US installation was on the [[New York Central Railroad]] (NYCRR) at Girard Junction, Ohio in 1937.<ref name="Alstom History" />{{rp|18}} Another NYCRR installation was on the main line between [[Utica, New York]] and [[Rochester, New York]], and this was quickly followed up by three installations on the [[New York City Subway]]'s [[IND Fulton Street Line]] in 1948.<ref name="RRSignal-INDPitkin-Sep1949">{{cite journal|title=Signaling and Interlocking On New Line of New York Subways|journal=Railway Signaling and Communications|date=September 1949|pages=578–583|url=https://books.google.com/books?id=EZHmAAAAMAAJ&pg=PA578|access-date=27 December 2016|publisher=Simmons-Boardman Publishing Corporation.}}</ref><ref name="NYTimes-INDEuclid-NX-1948">{{cite news|title=Buttons to Speed Travel in Subway: $2,000,000 System of Signals Soon to Be in Operation on Brooklyn IND Division|url=https://timesmachine.nytimes.com/timesmachine/1948/11/12/98429489.pdf|access-date=27 December 2016|work=[[The New York Times]]|date=November 12, 1948}}{{dead link|date=April 2019|bot=medic}}{{cbignore|bot=medic}}</ref>

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|Union Switch & Signal Co.]] (US&S), and introduced in 1951.<ref>{{cite patent|country=US|number=2567887|status=patent|title=Entrance-exit route interlocking control apparatus|invent1=Ronald A. McCann|gdate=11 September 1951|assign1=The Union Switch and Signal Company}}</ref> 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 ===
[[File:Antwerpen Noord seinhuis.jpg|thumb|Computer-based controls for a modern electronic interlocking]]

Modern interlockings (those installed since the late 1980s) are generally [[solid state (electronics)|solid state]], where the wired networks of relays are replaced by software logic running on special-purpose control hardware.<ref name="Solomon 2003"/>{{rp|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, this [[safety critical|vital]] logic is stored as [[firmware]] or in [[Read-only memory|ROM]] 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 with [[visual display unit]]s (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 generation [[microprocessor]]-based interlocking developed in the 1980s by [[British Rail]], GEC-General Signal and [[Westinghouse Rail Systems|Westinghouse Signals Ltd]] in the UK. Second generation processor-based interlockings are known by the term "Computer Based Interlocking" (CBI),<ref>{{cite report|last=Woolford|first=Paul|title=Glossary of Signalling Terms|url=http://author.rssb.co.uk/rgs/standards/GKGN0802%20Iss%201.pdf|access-date=20 April 2016|date=April 2004|publisher=Rail Safety and Standards Board|location=London|series=Railway Group Guidance Note GK/GN0802|url-status=dead|archive-url=https://web.archive.org/web/20160508205546/http://author.rssb.co.uk/rgs/standards/GKGN0802%20Iss%201.pdf|archive-date=8 May 2016}}</ref> of which [[Vital Processor Interlocking|VPI]] (trademark of [[General Railway Signal]], now Alstom), [[MicroLok]] (trademark of [[Union Switch & Signal]], now [[Hitachi Rail STS]]), [[Westlock Interlocking|Westlock]] and [[Westrace]] (trademarks of [[Invensys Rail Group|Invensys Rail]], now Siemens), [[Smartlock_(interlocking)|Smartlock]]<ref>{{cite web|url=http://www.alstom.com/products-services/product-catalogue/rail-systems/signalling/products/smartlock-interlocking-products/|title=Smartlock Interlocking|website=www.alstom.com|access-date=4 May 2018|url-status=live|archive-url=https://web.archive.org/web/20171008205925/http://www.alstom.com/products-services/product-catalogue/rail-systems/signalling/products/smartlock-interlocking-products/|archive-date=8 October 2017}}</ref> (trademark of [[Alstom]]), and [[EBI Lock]] (trademark of [[Bombardier Transportation|Bombardier]]) are examples.