Third Rail

Third rail is the electrified conductor laid alongside (or, less commonly, between) the running rails to deliver traction current to a train via sliding contact shoes mounted on the bogies. As an alternative to overhead catenary, it carries lower voltages — typically 600 to 1500 volts DC — but draws very high currents because power equals voltage times current. This trade-off makes it best suited to dense urban networks where infrastructure cost matters more than top speed and where overhead clearance is constrained by tunnels and bridges. The London Underground, New York Subway, Paris Métro, and large parts of Britain's Southern Region (750 V DC) all run on third rail.

The conductor rail itself is mounted on insulating pots on the sleepers, energised continuously between substations spaced every few kilometres. Top contact (shoe pressing down on the rail) is the simplest arrangement, used in London and on Southern; side contact (shoe pressing horizontally against the rail's flank) and bottom contact (shoe pressing up from underneath) trade complexity for better protection against rain, ice, and trespassers. Bottom contact is the de-facto standard on modern systems because the conductor can be sleeved in a plastic shroud, making accidental contact much harder.

Operationally, third rail forces a continuous-track regime: gaps are needed at switches and crossings, and trains must coast through them on stored momentum. Gap-jumping with multiple shoes per train solves part of the problem, but icing remains an annual headache that requires de-icing trains, anti-freeze paste, and in extreme cases shoe heaters on the rolling stock itself.

For railfans, third rail is what distinguishes the older European and American urban systems from their newer catenary-based light-rail competitors — and what makes any photograph from a third-rail platform a study in keeping your tripod legs well clear of the energised steel.