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 wheels bogies must be built to the same gauge; unless the two fit together within a 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 cross overs. For instance, 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, 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 be taken off one train and loaded on to the next.
In allowing railway tracks of different gauges to share the same alignment, costs can be reduced, especially where there are bridges and tunnels. Dual gauge can replace two separate tracks, having two rails each, with one track with three rails. This allows one rail fewer for the stretch of the dual gauge line, but there are complications and costs that may offset the savings.
One issue is points (US: switches). Complicated arrangements are necessary to ensure traffic of both gauges can safely utilise points. Signalling may also be complicated somewhat, as all three rails must be connected to track circuits or mechanical interlocking arrangements. Mixed gauge is simpler to signal with electric signals than with mechanical signals. Since rails wear very slowly, the extra tonnage on the common rail is generally not a problem.
Dual-gauge turnouts will be complicated, expensive, and suitable for low speeds only.
Read more about this topic: Dual Gauge
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