Editing Interlocking (section)

=== 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}}