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(last edited July 18, 2005) Front Page | Recent Changes | Title Index | User Preferences | Random Page | HelpAn additional rail (or rails) provided on those electric railways where power is transmitted to trains from the track. Often referred to as the 'third rail' or 'current rail' , it is normally at positive potential and is mounted on insulators to the outside of and slightly higher than the running rails. The return of the circuit is via the running rails. The current is collected by the train through 'shoes', attached to the bogies, which slide along or under the rail. Varieties of the system include top, side and bottom contact rails. Top contact rails are susceptible to ice and snow contamination in cold climates and present a certain risk to persons walking on the track. For this reason, bottom contact rails are preferred for modern systems. The continuity of conductor rails must be broken at junctions in the track to allow continuity of the running rails. Such 'gaps' may cause momentary loss of power to the train. There are cases from time to time of trains becoming 'gapped' at complex junctions, i.e. they stall over a gap and have to be rescued by another train. London Underground has a fourth rail (negative) for a completely insulated circuit. It is known as a four rail system.
