For the purposes of this article the generic term overhead line has been used.
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.
Electric trains that collect their current from overhead line system use a device such as pantograph, bow collector, or trolley pole. The current collection device presses against the underside of the lowest wire of an overhead line system, which is called a contact wire. The current collectors are electrically conductive, and allow current to flow through to the transformer of the train or tram, and back to the feeder station through the steel wheels and one or both running rails of the track. Diesel trains may pass along these tracks without affecting the overhead line, although overhead clearance may be an issue.
To achieve good high speed current collection, it is necessary to keep the contact wire geometry within defined limits throughout the length of the overhead line. It is usually achieved by supporting the contact wire from above by means of a second wire, known variously as the messenger wire (US & Europe) or catenary (UK & Canada). This wire is allowed to follow the natural path of a wire strung between two points, which is known as a catenary curve, thus the use of catenary to describe this wire or sometimes the whole system. This wire is attached to the contact wire at regular intervals by vertical wires known as droppers or drop wires. In this way the contact wire is effectively supported at numerous points. The messenger wire is supported regularly at structures, either by means of a pulley, link, or clamp. The whole system is then subjected to a mechanical tension.
As the contact wire makes contact with the pantograph, the carbon surface of the insert on top of the pantograph is worn down. At high speeds the effect will wear a groove or notch at a single location in the centre of the pantograph wiper. Going around a curve, the "straight" wire (between supports) will cause the contact wire to cross over the whole surface of the pantograph wiper as the train travels around the curve, causing an even wear and avoiding any notice. On straight track, the contact wire is intentionally zigzagged slightly to the left and right of centre at each successive support. This horizontal movement is introduced so that the pantograph wears evenly as the vehicle moves in the direction of travel.
The zigzagging of the overhead line was not required on older trolley-based trams, or for trolley-buses. There is no wiper, or 'bow' involved with all the friction wear being taken up by the pulley and bearing at the end of the trolley pole.
Tram, trolley and depot areas tend to only have a single wire and are known as simple equipment. When overhead line systems were first conceived, good current collection was only possible at low speeds using the single supporting wire system. To enable higher-speed collection, two additional types of equipment were developed to improve contact reliability:
Dropper wires traditionally only provide physical support of the contact wire, and do not join the catenary and contact wires electrically. Contemporary systems use current-carrying droppers, which eliminate the need for separate wires.
For tramways there is often just a simple contact wire and no messenger wire.
In Germany there are special overhead power lines for single phase AC traction current with a frequency of 16.7 hertz. Most of these lines, which are all operated with a voltage of 110 kV (the voltage of the supply cables, not the voltage of the railway overhead lines which is 15 kV) have four conductor cables for two circuits. As a rule at traction current lines, the single-level arrangement of the conductor cables is used. A traction current pylon is a pylon with at least carry one electric circuit for traction current. For traction current lines with four circuits (eight conductor cables) frequently two-level arrangements of conductors are used, at which one crossbar carries four conductor cables. For traction current lines used for supplying new high-speed rail tracks, three-level arrangements of conductors are used. Thereby are on the lowest crossbar four, and on the upper crossbars two, conductor cables mounted. The three-level arrangement is also used for traction current lines with 6 electric circuits (12 conductor cables).
There are further, in particular for the power supply of rapid transit railways operated with alternating current, overhead line pylons with crossbars for 110 kV traction current powerlines above the contact wire in use. There are also pylons that carry electric circuits for traction current and for three-phase alternating current of the public power grid.
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 each end, the whole tension length will be free to move along track. Therefore, a midpoint anchor (MPA) is introduced close to the centre of the tension length to restrict movement. MPAs are often fixed to low bridges.
Therefore a tension length can be seen as a fixed centre point with the two half tension lengths expanding and contracting with temperature.
To allow maintenance to sections of the overhead line without having to turn off the entire system, the overhead line system is broken into electrically separated portions known as sections. Sections often correspond with tension lengths as described above. The transition from section to section is known as a section break and is set up so that the locomotive's pantograph is in continuous contact with the wire.
For bow collectors and pantographs, this is done by having two contact wires run next to each other over a length about four wire supports: a new one dropping down and the old one rising up until the pantograph smoothly transfers from one to the next. The two wires never touch (although the bow collector/pantograph is briefly in contact with both wires). In normal service the two sections are electrically connected (to different substations if at or near the halfway mark between them), but this can be broken for servicing.
On overhead wires designed for trolley poles this is done by having a neutral section between the wires, but this requires an insulator. The driver of the tram or trolleybus must turn off the power when the trolley pole passes through to prevent arcing causing damage to the insulator.
Sometimes on a larger electrified railway, tramway or trolleybus system it is necessary to power different areas of track from different power grids, the synchronisation of the phases of which cannot be guaranteed. (Indeed, sometimes the sections are even powered with different voltages or frequencies.) There may be mechanisms for having the grids synchronised on a normal basis, but events may cause desynchronisation. This is no problem for DC systems, but for AC systems it would be quite undesirable to connect two unsynchronised grids together, even momentarily. A normal section break is insufficient to guard against this since the pantograph briefly connects both sections.
Instead, a phase break or neutral section is used. This consists of two section breaks back-to-back so that there is a short section of overhead line that belongs to neither grid. If the two grids are synchronised, this stretch of line is energised (by either supply) and trains run through it normally. If the two supplies are not synchronised, the short isolating section is disconnected from the supplies, leaving it electrically dead, ensuring that the two grids cannot be connected to each other.
The sudden loss of power over the phase break would jar the train if the locomotive was at full throttle, so special signals are set up to warn the crew. When synchronization is lost and the phase break is deenergised, the train's operator must put the controller (throttle) into neutral and coast through an isolated phase break section.
On the Pennsylvania Railroad, phase breaks were indicated to train crews by a metal sign hung from the overhead with the letters PB on it, created by holes drilled in the metal. When the phase break was "dead", a signal consisting of eight lit lights in a circular pattern indicated this to the crew.
The South African state owned transport company, Transnet Freight Rail, has permanent magnets installed between the rails at both sides of the neutral section where two phases are separated. These magnets are detected by equipment on the locomotives and disconnect power from the pantographs and switches it back on detecting the outgoing magnet.
Trams draw their power from a single overhead wire at about 500 to 750 V above earth, while trolleybuses draw their power from two overhead wires (powered at similar voltage). Because of that, at least one of the trolleybus wires must be insulated from tram wires. This is usually solved in the following way: the trolleybus wires run continuously through the crossing. The tram conductors are slung a few centimetres lower than the trolleybus wires. Close to the junction on each side, the wire merges into a solid bar which is angled to run parallel to the trolleybus wires for about half a metre. Another bar similarly angled at its ends is hung between the trolleybus wires. This is electrically connected above to the tram wire's catenary cable. The tram's pantograph bridges the gap between the different conductors, providing it with a continuous pickup.
Where the tram wire crosses, the trolleybus wires are protected by an inverted trough of insulating material extending 20 or 30 mm below the level of the trolleybus wires. The tram pantograph or bow collector raises the conductor wire a little as it passes under. These troughs are presumably to limit how far it can do that and to provide a backstop to prevent the tram pantograph or bow collector ever touching the trolleybus wires.
Several cities use the above system. Until 1946 there was a level crossing in Stockholm, Sweden between the railway south of Stockholm Central Station and a tramway line. The tramway operated on 600-700 V DC and the railway operated on 15 kV AC. Some crossings between tramway/light rail and railways are still alive in Germany. In Zürich, Switzerland the VBZ trolleybus line 32 has a level crossing with the 1200V DC railway to mount Uetliberg; at many places in the town trolleybus lines cross the tramway. In the Swiss village Suhr the tramway WSB operating at 1200V DC crosses the SBB line on 15 kV AC. In some cities, trolleybuses and trams have shared the same positive (feed) wire. In such cases a normal trolleybus frog can be used.
Another system that has been used is to coincide section breaks with the crossing point so that the crossing is electrically dead.
Many cities had trams and trolleybuses both using trolley pole current collection. They used insulated crossovers which required tram drivers to put the controller into neutral and coast through. Trolleybus drivers had to either lift off the accelerator or switch to auxiliary power.
In Melbourne, Victoria tram drivers are still required to put the controller into neutral and coast through section insulators, this being indicated to drivers by insulator markings between the rails.
Melbourne also has another interesting issue - crossings between electrified suburban railways and tram lines at grade. There are four of these level crossings through the systems and each requires complex switching arrangements to separate the operation of 1500 V DC overhead for the railway and 650 V DC for the trams. This is called an overhead square. Proposals have been put forward which would eventually see most or all of these crossings grade separated or the tram routes diverted.
Queensland uses a 25,000 V overhead Traction system, a booster transformer system in the suburban network and an auto transformer network in the larger expanses of the state.
In Athens there are two crossings between overhead tram and trolleybus wires. These crossings are at the junction of Vas. Amalias Avenue with Vas. Olgas Avenue, and the junction of Ardittou Street with Athanasiou Diakou Street. They use the above-mentioned solution for crossing tram and trolleybus wires.
Additionally, for about a year (from the opening of the tram system in the summer of 2004 until mid-2005), trams and trolleybuses going in the direction of Pagrati shared the same exclusive lane on the far right side of Vas. Olgas Avenue (which is about 400 m long); this required tram and trolleybus wires to coexist side-by-side above a very narrow lane of road. To solve this problem, the trolleybus wires were placed on the far right of the lane, rendering it impossible for the tram's (very wide) pantograph to come into contact with them. As a side-effect, however, trolleybus drivers were required to employ much greater caution on this stretch of road, and drive very slowly through Vasilisis Olgas Avenue, owing to the trolleybus collectors being extended to their limits under this arrangement. Finally, a change of route for trolleybuses was implemented in mid-2005, avoiding Vas. Olgas Avenue completely, and ending this difficult coexistence.
There are and were some railways which used two and even three overhead lines, usually to carry three-phase current to the trains. Nowadays, three phase AC current is used only on the Gornergrat Railway and Jungfraujoch Railway in Switzerland, the Petit train de la Rhune in France, and the Corcovado Rack Railway in Brasil; until 1976, it was widely used in Italy. On these railways the two conductors of the overhead lines are used for two different phases of the three-phase AC, while the rail was used for the third phase. The neutral was not used.
Some three-phase AC railways used three overhead wires. These were an experimental railway line of Siemens in Berlin-Lichtenberg in 1898 (length: 1.8 kilometres), the military railway between Marienfelde and Zossen between 1901 and 1904 (length: 23.4 kilometres) and an 800-metre-long section of a coal railway near Cologne, between 1940 and 1949.
On DC systems bipolar overhead lines were sometimes used to avoid galvanic corrosion of metallic parts near the railway. An example of a railway run with DC using two overhead lines was the Chemin de fer de la Mure.
All systems of multiple overhead lines have the disadvantage of high risk of short circuits at switches and therefore tend to be impractical in use, especially when high voltages are used or when trains run through the points at high speed.
Unlike simple overhead wires, in which the uninsulated wire or cable is attached by clamps to closely spaced crosswires, themselves supported by line poles, catenery system use at least two wires. One wire, called the catenary wire or the messenger wire, is hung at a specific tension value in the shape of a mathematical catenary between line structures. A second wire is held in tension by the messenger wire, to which it is attached at frequent intervals by clamps and connecting wires. The second wire is straight and level, parallel to the rail tracks, suspended over it as the roadway of a suspension bridge is over water.
The Northeast Corridor in the United States features electrified catenary over a 600-mile or 1000 km distance between Boston, Massachusetts and Washington, D.C., providing power for Amtrak's high-speed Acela Express and other trains. Several commuter rail agencies, including MARC, SEPTA, NJ Transit, Metro-North utilize the catenary to provide local service along the Northeast Corridor.
The interurban rapid transit situation in Cleveland, Ohio is a little out of the ordinary. The interurban/light rail system uses overhead lines as usually expected. However, the heavy rail system also uses overhead lines instead of a third rail. Historically this was due to a city ordinance intended to limit air pollution from the large number of steam locomotive trains passing through the Cleveland en route between East coast cities and Chicago. Trains would switch from steam to Overhead catenary electric locomotives at the Collinwood Rail Yards about east of Downtown Cleveland, and also similarly on the Cleveland's West side, or else reroute outside of city incorporation limits to the south. When Cleveland constructed their rapid transit (heavy rail) line between the airport, Downtown Cleveland, and beyond they employed similar overhead catenary technologies that the railroads used, and were able to utilize railroad electrification surpluses left over after railroads switched from steam to diesel locomotives. Consequently, both light and heavy rail public transit systems are able to share trackage for about along the Cleveland Hopkins International Airport Red (heavy rail) line, Blue and Green interurban/light rail lines between Cleveland Union Terminal and just past the East 55th. Street station where the heavy- and light-rail line tracks separate.
The height of overhead wiring can create hazards at level crossings, where it may be struck by road vehicles. The wiring in most countries is too low to allow Double stack container trains. India is proposing a network of freight only lines, which would almost certainly be electrified with extra height wiring and pantographs that can reach this height.