Pantograph (rail)

A pantograph is the name commonly given to the arms that collect current from overhead lines on electric trains or trams. The term originates in the similar appearance they share with the earlier pantograph devices for copying writing and drawings, both being constructed of hinged interlocking arms.

Related Topics:
Overhead lines - Train - Tram - Pantograph devices

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The electricity transmission system for modern electric rail systems consists of an upper load carrying wire (known as a catenary) from which is suspended a contact wire. The pantograph is spring loaded and pushes a contact shoe up against the contact wire to draw the electricity needed to run the train. The steel rails on the tracks act as the electrical return.

Related Topics:
Catenary - Electrical return

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As the train moves, the contact shoe slides along the wire and can set up standing waves in the wires which break the contact and degrade current collection. This means that on some systems adjacent pantographs are not permitted. More recent systems use roller-type contact shoes which reduce this problem. Pantographs are the successor technology to trolley poles, which were widely used on early streetcar systems and still are used by trolleybuses, whose freedom of movement and need for a two-wire circuit makes pantographs impractical, and by some streetcar systems, although in most cases only heritage lines which use old-fashioned vehicles.

Related Topics:
Standing wave - Trolley pole - Trolleybus - Streetcar - Heritage lines

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Pantographs with overhead wires are now the dominant form of current collection for modern electric trains, because while they are more expensive and fragile than a third-rail system, they also allow for higher voltages.

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