Editing Overhead line (section)

== Overhead conductor rails ==
[[File:B&O electrification.jpg|thumb|left|B&O's overhead third-rail system at [[Guilford Avenue]] in Baltimore, 1901, part of the [[Baltimore Belt Line]]. The central position of the overhead conductors was dictated by the many tunnels on the line: the '''∩'''-shaped rails were located at the highest point in the roof to give the most clearance.<ref name=journal>{{cite journal |date=10 August 1895 |title=A ninety-six ton electric locomotive |journal=[[Scientific American]]|location=New York}}</ref>]]

Given limited [[Engineering tolerance#Clearance (civil engineering)|clearance]] such as in [[tunnel]]s, the overhead wire may be replaced by a rigid overhead rail. An early example was in the tunnels of the [[Baltimore Belt Line]], where a '''Π''' section bar (fabricated from three strips of iron and mounted on wood) was used, with the brass contact running inside the groove.<ref name=journal/> When the overhead line was raised in the [[Simplon Tunnel#Piggyback transport|Simplon Tunnel]] to accommodate taller rolling stock, a rail was used. A rigid overhead rail may also be used in places where tensioning the wires is impractical, for example on [[moveable bridges]]. In modern uses, it is very common for underground sections of trams, metros, and mainline railways to use a rigid overhead wire in their tunnels, while using normal overhead wires in their above ground sections.

[[File:Shaw's Cove Railroad Bridge closing.webm|thumb|Operation of the overhead conductor rails at Shaw's Cove Railroad Bridge in Connecticut]] 
In a movable bridge that uses a rigid overhead rail, there is a need to transition from the catenary wire system into an overhead conductor rail at the bridge portal (the last [[traction current pylon]] before the movable bridge). For example, the power supply can be done through a catenary wire system near a [[swing bridge]]. The catenary wire typically comprises messenger wire (also called catenary wire) and a contact wire where it meets the pantograph. The messenger wire is terminated at the portal, while the contact wire runs into the overhead conductor rail profile at the transition end section before it is terminated at the portal. There is a gap between the overhead conductor rail at the transition end section and the overhead conductor rail that runs across the entire span of the swing bridge. The gap is required for the swing bridge to be opened and closed. To connect the conductor rails together when the bridge is closed, there is another conductor rail section called "rotary overlap" that is equipped with a motor. When the bridge is fully closed, the motor of the rotary overlap is operated to turn it from a tilted position into the horizontal position, connecting the conductor rails at the transition end section and the bridge together to supply power.<ref>{{cite book |last1=Cox |first1=Stephen G. |last2=Nünlist |first2=Felix |last3=Marti |first3=Reto |title=Electrification of swing and bascule bridges with overhead conductor rails |date=25 September 2000 |publisher=Northend Electrification Project |pages=3–4 |url=http://www.bahnjournalisten.ch/files/anlaesse/referate/Furrer_Frey_Swingbridges_CT.pdf |access-date=25 June 2018}}</ref>

Short overhead conductor rails are installed at [[tram stop]]s as for the [[Combino Supra#Doha|Combino Supra]].<ref>[http://www.siemens.com/press/en/pressrelease/?press=/en/pressrelease/2012/infrastructure-cities/rail-systems/icrl201207010.htm Siemens press release]</ref>