A third rail is a method of providing electricity to power a railway through a continuous rigid conductor alongside the railway track or between the rails. It is used typically in a mass transit or rapid transit system, which has alignments in own corridors, fully or almost fully segregated from the outside environment. A list of lines or networks equipped with a third rail is below. Third-rail systems generally supply direct current to power the trains. In the early 1900s the third-rail system was used to power early rollercoasters such as the Rough Riders in Coney Island, New York.
Third-rail electric systems are, apart from on-board batteries, the oldest means of supplying electric power to trains on railways using own corridors, particularly in cities. Overhead power supply was initially almost exclusively used on tramway-like railways, though it also appeared slowly on mainline systems. (This statement describes the general trend; early particular cases may have been different.)
An experimental electric train using this method of power supply was developed by the German firm of Siemens & Halske and shown at the Berlin Industrial Exhibition of 1879. This pioneer electric railway had its third rail placed between running rails. At some early electric railways, though, one of the running rails could be the current conductor, as was the case of the 1883-opened Volk's Electric Railway in Brighton. Soon it was given an additional power rail in 1886 (the railway is still operating). The Giant's Causeway Tramway followed, equipped with an elevated outside third rail in 1883 (but later converted to overhead wire pickup). The first railway to use the central third rail was the Bessbrook & Newry Tramway, opened in Ireland in 1885 but now, like the Giant's Causeway line, closed. Also in the 1880s third-rail systems began to be used in public urban transport. Trams were first to benefit from it, but they used conductors built in conduit below the road surface (cf. Conduit current collection), and usually on selected parts of the networks. This was first tried in Cleveland (1884) and in Denver (1885) and later spread to many big tram networks (e.g. Manhattan, Chicago, Washington DC, London, Paris — all closed) and Berlin (The third rail System in the city was abandoned in the first years of the twenties century — after heavy snowfall)) .
A third rail supplied power to the world's first electric underground railway, the City & South London Railway, which opened in 1890 (now part of the Northern Line of the London Underground). In 1893 the world's second third-rail powered city railway opened in Britain — the Liverpool Overhead Railway (closed 1956 and dismantled). The first US third-rail powered city railway in revenue use was the 1895-opened Metropolitan West Side Elevated, which soon became part of the Chicago 'L'. In 1901, Granville Woods, a prominent African-American inventor, was granted a Template:US patent, covering various proposed improvements to third rail systems. This has been cited to claim that he invented the third rail system of current distribution. However, by that time there had been numerous other patents for electrified third-rail systems, including Thomas Edison's Template:US patent of 1882, and third rails had been in successful use for over a decade, in installations including the rest of Chicago 'elevateds', as well as these in Brooklyn, New York (if not to mention the development outside the US). To what extent Woods' ideas were adopted is thus a matter of controversy.
In Paris, in 1900, third rail appeared in the mainline tunnel connecting the Gare d'Orsay to the rest of the CF Paris-Orléans network. Mainline third rail electrification was later expanded to some suburban services in the French capital.
Top contact third rail (cf. below) seems to be the oldest form of power collection. Railways pioneering in using other, less hazardous types of third rail, were the New York Central Railroad on the approach to its NYC's Grand Central Terminal (1907 — another case of a third-rail mainline electrification) and the Hochbahn in Hamburg (1912) — both had bottom contact rail. However, the Manchester-Bury Line of the Lancashire & Yorkshire Railway tried the side contact rail (1917). These technologies appeared in wider use only at the turn of the 1920s and in the 1930s at, e.g., large-profile lines of the Berlin U-Bahn, the Berlin S-Bahn and the Moscow Metro. The Hamburg S-Bahn is using a side contact rail with 1200 V dc since 1939.
In 1956 world's first rubber-tired railway line was opened. This was Line 11 of Paris Metro. Power rail evolved into a pair of guiding rails required to keep the bogie in proper position on the new type of track. This solution was modified on the 1971-opened Namboku Line of Sapporo Subway, where a centrally placed guiding/return rail was used plus one power rail placed laterally as usually on steel rail railways (cf. photo).
The third rail technology at street tram lines has recently been revived in the new system of Bordeaux (2004). This is a completely new technology (cf. below).
Third rail, being the older of the two electric current supply methods, is by no means obsolete. There are, however, countries (particularly Japan, South Korea, India, Spain) more eager to adopt overhead wiring to their urban railways. But in the same time there were (and still are) many new third rail systems built elsewhere, including technologically advanced countries (i.e. Copenhagen Metro, Taipei Metro, Wuhan Metro). Bottom powered railways (it may be too specific to use the term 'third rail') are also usually these having rubber-tyred trains, no matter if it is a heavy metro (except two other lines of Sapporo Subway) or a small capacity people mover (PM). Practically the only type of railways where third rail is no longer used in new systems is regional and long distance rail, which require higher speeds and voltages.