Editing Overhead line (section)

=== Parallel overhead lines ===
[[File:Catenaryswitch.jpg|thumb|A switch in parallel overhead lines]]
[[File:Trolley3WireSwitch.jpg|thumb|left|[[Trolleybus#Wire switches|Trolleybus wire switch]]]]

An electrical circuit requires at least two conductors. Trams and railways use the overhead line as the positive terminal of the circuit and the steel rails as the negative terminal of the circuit. For a [[trolleybus]] or a [[trolleytruck]], no [[Rail profile|rails]] are available for the return current, as the vehicles use rubber tyres on the road surface. Trolleybuses use a second parallel overhead line for the return, and two [[trolley pole]]s, one contacting each overhead wire. ([[Pantograph (transport)|Pantographs]] are generally incompatible with parallel overhead lines.) The circuit is completed by using both wires. Parallel overhead wires are also used on the rare railways with [[three-phase AC railway electrification]].