Overhead line

Electric trains that collect their current from overhead lines use a device such as a pantograph, bow collector or trolley pole.

On straight track, the contact wire is zigzagged slightly to the left and right of the centre from each support to the next so that the insert wears evenly, thus preventing any notches.

When overhead line systems were first conceived, good current collection was possible only at low speeds, using a single wire.

A somewhat higher tension than used before clipping the beam yielded a deflected profile for the wire that could be easily handled at 400 km/h (250 mph) by a pneumatic servo pantograph with only 3 g acceleration.

Common materials include normal and high strength copper, copper-silver, copper-cadmium, copper-magnesium, and copper-tin, with each being identifiable by distinct identification grooves along the upper lobe of the contact wire.

These devices contain a torsional spring with a cam arrangement to ensure a constant applied tension (instead of varying proportionally with extension).

[6] An additional issue with AT equipment is that, if balance weights are attached to both ends, the whole tension length is free to move along the track.

German systems usually use a single large tensioning pulley (basically a ratchet mechanism) with a toothed rim, mounted on an arm hinged to the mast.

In normal service, the two sections are electrically connected; depending on the system this might be an isolator, fixed contact or a Booster Transformer.

The driver of the tram or trolleybus must temporarily reduce the power draw before the trolley pole passes through, to prevent arc damage to the insulator.

[7] In countries such as France, South Africa, Australia and the United Kingdom, a pair of permanent magnets beside the rails at either side of the neutral section operate a bogie-mounted transducer on the train which causes a large electrical circuit-breaker to open and close when the locomotive or the pantograph vehicle of a multiple unit passes over them.

The position light signal aspect was originally devised by the Pennsylvania Railroad and was continued by Amtrak and adopted by Metro North.

Metal signs were hung from the catenary supports with the letters "PB" created by a pattern of drilled holes.

Since its traction power network was centrally supplied and only segmented by abnormal conditions, normal phase breaks were generally not active.

Phase breaks that were always activated were known as "Dead Sections": they were often used to separate power systems (for example, the Hell's Gate Bridge boundary between Amtrak and Metro North's electrifications) that would never be in-phase.

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.

[13] A catenary is a system of overhead wires used to supply electricity to a locomotive, tram (streetcar), or light rail vehicle that is equipped with a pantograph.

The second wire is straight and level, parallel to the rail track, suspended over it as the roadway of a suspension bridge is over water.

The Northeast Corridor in the United States has catenary over the 600 miles (970 km) between Boston, Massachusetts and Washington, D.C., for Amtrak's inter-city trains.

Commuter rail agencies including MARC, SEPTA, NJ Transit, and Metro-North Railroad utilize the catenary to provide local service.

In Cleveland, Ohio, the interurban/light rail lines and the heavy rail line use the same overhead wires, due to a city ordinance intended to limit air pollution from the large number of steam trains that passed through Cleveland between the east coast and Chicago.

Trains switched from steam to electric locomotives at the Collinwood railyards about 10 miles (16 km) east of Downtown and at Linndale on the west side.

When Cleveland constructed its rapid transit (heavy rail) line between the airport, downtown, and beyond, it employed a similar catenary, using electrification equipment left over after railroads switched from steam to diesel.

The Channel Tunnel has an extended height overhead line to accommodate double-height car and truck transporters.

China and India operate lines electrified with extra height wiring and pantographs to allow for double stack container trains.

Also, the added construction and maintenance cost-per-mile makes overhead systems less attractive on already existing long-distance railways, such as those found in North America, where the distances between cities are typically far greater than in Europe.

Such long lines require enormous investment in overhead line equipment, which private rail companies are unlikely to be interested in, and major difficulties confront energizing long portions of overhead wire on a permanent basis, especially in areas where energy demand already outstrips supply.

Many people consider overhead lines to be "visual pollution", due to the many support structures and complicated system of wires and cables that fill the air.

The issue came to a head in the UK with the Great Western Main Line electrification scheme, especially through the Goring Gap.

The first tram with overhead lines was presented by Werner von Siemens at the 1881 International Exposition of Electricity in Paris: the installation was removed after that event.