circa 1895: A view of Canal Street, with trolleys running on overhead lines and horsedrawn carriages, New Orleans, Louisiana, 1890s. A sign for the Orpheum Theatre hangs in the street. Several men are gathered by a street lamp. (Photo by P. L. Sperr/Hulton Archive/Getty Images)


Overhead lines or overhead wires are used to transmit electrical energy to trams, trolleybuses or trains at a distance from the energy supply point. These overhead lines are known variously as

  • Overhead contact system (OCS) – Europe, except UK
  • OverHead line equipment (OLE or OHLE) – UK
  • Overhead wiring (OHW) – Australia
  • Catenary – United States and Canada.

Overhead line is designed on the principle of one or more overhead wires 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.


Catenary wires are kept at a mechanical tension because the pantograph causes oscillations in the wire, and the wave must travel faster than the train to avoid producing standing waves that would cause the wires to break. Tensioning the line makes waves travel faster.

For medium and high speeds the wires are generally tensioned by means of weights, or occasionally by hydraulic tensioners. Either method is known as auto-tensioning (AT), and ensures that the tension in the equipment is virtually independent of temperature. Tensions are typically between 9 and 20 kN per wire.

For low speeds and in tunnels where temperatures are constant, fixed termination (FT) equipment may be used, with the wires terminated directly on structures at each end of the overhead line. Here the tension is generally about 10 kN. This type of equipment will sag on hot days and hog on cold days.

Where AT is used, there is a limit to the continuous length of overhead line which may be installed. This is due to the change in the position of the weights with temperature as the overhead line expands and contracts. This movement is proportional to the tension length, i.e. the distance between anchors. This leads to the concept of maximum tension length. For most 25 kV OHL equipment in the UK the maximum tension length is 1970 m.

An additional issue with AT equipment is that if balance weights are attached to both ends the whole tension length will be free to move along track. Therefore, a midpoint anchor (MPA) close to the center of the tension length restricts movement. MPAs are often fixed to low bridges. Therefore, a tension length can be seen as a fixed center point with the two half tension lengths expanding and contracting with temperature.