Overhead wires

An overhead line or overhead wire is used to transmit electrical energy to trams, trolleybuses, or trains. It is known variously as:

In this article, the generic term overhead line is used, as used by the International Union of Railways.

Overhead line is designed on the principle of one or more overhead wires (or rails, particularly in tunnels) situated over rail tracks, raised to a high electrical potential by connection to feeder stations at regular intervals. The feeder stations are usually fed from a high-voltage electrical grid.

Electric trains that collect their current from overhead lines use a device such as a pantograph, bow collector or trolley pole. It presses against the underside of the lowest overhead wire, the contact wire. Current collectors are electrically conductive and allow current to flow through to the train or tram and back to the feeder station through the steel wheels on one or both running rails. Non-electric locomotives (such as diesels) may pass along these tracks without affecting the overhead line, although there may be difficulties with overhead clearance. Alternative electrical power transmission schemes for trains include third rail, ground-level power supply, batteries, and electromagnetic induction.

This article does not cover regenerative braking, where the traction motors act as generators to retard movement and return power to the overhead.

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