Pantograph (rail)

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A pantograph is the name commonly given to the arms that collect current from Overhead Lines on electric Train s or Tram s. The term originates in the similar appearance they at first they shared with the earlier Pantograph Devices for copying writing and drawings, both being constructed of hinged interlocking arms. Today, the most common type is the so called half-panto (sometimes 'Z'-shaped), with only one pair interlocking arms, which is simpler to construct but is not generally as robust.

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 .

As the train moves, the contact shoe slides along the wire and can set up Standing Wave s 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 Pole s, which were widely used on early streetcar systems and still are used by Trolleybus es, 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. A notable exception is Toronto , Canada . Until recently, Melbourne was the largest surviving exception.

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.

LIST OF METRO SYSTEMS USING PANTOGRAPHS

Most Rapid Transit systems are powered by a Third Rail , but some use pantographs, particularly ones that involve extensive above-ground running. (Hybrid metro-tram or 'pre-metro' lines whose routes include tracks on city streets or in other publicly-accessible area must of course use overhead wire, since a third rail would normally present too great a risk of electrocution.) The only current exception to this is the new Bordeaux tram system which uses a system called Alimentation Par Sol which only applies power to segments of track that are completely covered by the train. This system is used in the historic centre of Bordeaux where an overhead wire system would cause a visual intrusion.

Among those electrified railway systems which use pantographs are the following: