Editing Overhead line (section)

=== Electrification support structures ===
Overhead line equipment may be supported above the running lines by a range of different methods. Support structures are often designed to allow mechanically independent registration (MIR) which refers to a setup where each contact and catenary wire for each track is mechanically independent from the adjacent wire runs. Support structures are required to provide ''support'' and ''registration'' to the OLE wires. In OLE terminology, ''support'' refers to vertical position of catenary wire (and of the contact wire via the catenary wire), while ''registration'' refers to the horizontal position of both catenary and contact wires. The metal parts the provide OLE registration are typically designed to adjust in the vertical plane as the pantograph moves under it. The generic types of support structures are summarised below.<ref>{{Cite book |last=Keenor |first=Garry |url=https://ocs4rail.com |title=Overhead Line Electrification for Railways |year=2021 |edition=6th |pages=136–147 |format=pdf e-book, occasionally updated}}</ref>

[[File:Nanded - Hyderabad Passenger Cavalry Barracks.jpg|thumb|145px|right|Gantry structures to support overhead lines in [[Hyderabad]], India]]
* Single Cantilever
** The most basic and common type of OLE structure that supports and registers one wire run above one track.
* Double Cantilever
** Similar to the previous but with two cantilever arms adjacent to each other on one mast. Often used where two wire runs converge.
* Back to Back Cantilever
** Two wire runs over two tracks supported and registered by one mast placed in the centre of the tracks with cantilever arms attached to opposite sides. Frequently used on tram and light rail systems but can appear on heavy rail lines.
* Two Track Cantilever (TTC)
** Two wire runs over two tracks supported by one mast with a boom structure extending over to the second track. TTCs typically provide mechanically independent registration, but subtypes exists called the "span-wire two track cantilever" which has both registration arms mechanically linked. TTCs are often used  where there are poor ground conditions or obstructions on one side of a two track railway. They are also sometimes used to minimised piling since only one track must be taken out of service for this phase of construction.
* Portals
** Also known generically as "gantries", portals are large structures with masts on either side and a fixed steel beam between them. They are frequently used on sections of railway line with more than two tracks. Portals typically provide mechanically independent registration, however variants exist with registration span-wires that mechanically link adjacent wire runs. Portals are sometimes used on two track railways over bridges and viaducts or where ground conditions are poor. This is because portals inflict fewer rotational forces on their foundations.
* Headspans
** An alternative method to supporting multi-track areas, headspans consist of a mast at either side of the railway and various cables running horizontally between the two masts (called span wires) to support and register all wire runs at tension. Because the tension of all wire runs and span wires are required simultaneously to hold up the OLE wires, headspans by definition do not provide mechanically independent registration and a failure of one OLE or span wire will bring all wire runs out of geometric limits. Headspans are cheaper and less obtrusive than portals and so are well suited to low speed complex multitrack areas like stations, station approaches, depots and sidings, and areas where visual intrusion is an important consideration. They are also capable of supporting tracks with speeds up to 200km/h (125mph) but provide significant reliability disadvantages over portals or TTCs for this application.
Tunnels, low overbridges and other location specific features (retaining walls, adjacent rockfaces etc) frequently require bespoke OLE structures that may incorporate some features of the generic types above.