Editing Interlocking (section)

=== 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.