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dual gauge
A dual-gauge railway is a line of track that provides for trains of two separate 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 provide 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.
== Reasoning ==
In railways, the most important specification is that of gauge, the distance between the inner surfaces of the heads of the travel rails (see diagram above). Both track and wheel bogies must be built to the same gauge; unless the two fit together within a typical tolerance of 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 -gauge EMUs may run on KCR -gauge rails, with a locomotive or a KCR EMU pulling due to different electrification voltages. A problem arises when different gauges meet one another, a situation known as a break-of-gauge. Either the track or the train must be built to handle different gauges, or passengers and freight must transfer between different trains, or in some instances, the rolling stock is lifted and the bogies are removed and replaced with different bogies for the other gauge.
In allowing railway tracks of different gauges to share the same alignment, costs can be reduced, and infrastructure shared, e.g. platforms, bridges and tunnels etc. Dual gauge can replace two separate tracks by sharing one common running rail and one of the two other outer rails which determine the gauge.
However, there are some complications and costs involved that may offset the savings. One issue is points (US: switches) are more complex, and therefore more expensive. Arrangements are necessary to ensure trains of both gauges can be safely signalled, track circuits and mechanical interlocking where provided must be operated through the common rail. Also, the common rail will have an increased wear and tear over the other separate gauge rails.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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