Mixed gauge
A dual-gauge railway is a line of track that provides for trains of two separate track gauges. A mixed-gauge railway provides for more than two separate gauges, but is also a term sometimes used to denote dual-gauge.
A dual-gauge track configuration usually consists of three rails, two 'vital' (or gauge-) rails, one for each gauge, plus a 'common rail'--although at times, commonality is not possible and four rails are required (the two separate tracks of each gauge are placed within each other). In an ordinary three-rail dual-gauge configuration, the two adjacent outer rails give each of the gauges, while the single outer rail is common to trains of both gauges operating over it. This configuration is not to be confused with the electric current traction rail (third rail) or a check or guard rail.
In railways, the most important specification is that of rail gauge, the distance between the inner surfaces of the heads of the travel rails. Both track and wheel bogies must be built to the same gauge; unless the two fit together within a typical tolerance of 13 mm (0.5 in) on the track, the train will either fall off the track or it will be impossible to go through switches or crossovers. However, there is a small tolerance; for example, the Hong Kong MTR 1,432 mm (4 ft 8 3⁄8 in) gauge EMUs may run on KCR 1,435 mm (4 ft 8 1⁄2 in) gauge rails (but not vice versa), with a locomotive or a KCR EMU pulling due to different electrification voltages. Another example would be Finno-Russian border: when in the Seventies the Soviet Union redefined the breadth of a Russian gauge from the traditional five feet (1524 mm) to exactly 1520 mm, the Finnish railways stuck with the old value. This did not result in the break-of-gauge because no actual track work was done, as the redefinition was more of a change in the way the tolerances were counted, both railways remained well within tolerances of each other, and several years later Finland also tightened tolerances much the same way as the USSR did, but without the change to the nominal gauge.
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