In railway engineering, "gauge" is the transverse distance between the inner surfaces of the heads of two rails, which for the vast majority of railway lines is the number of rails in place. However, it is sometimes necessary for track to carry railway vehicles with wheels matched to two different gauges. Such track is described as dual gauge – achieved either by addition of a third rail, if it will fit, or by two additional rails. Dual-gauge tracks are more expensive to configure with signals and sidings, and to maintain, than two separate single-gauge tracks. It is therefore usual to build dual-gauge or other multi-gauge tracks only when necessitated by lack of space or when tracks of two different gauges meet in marshalling yards or passenger stations. Dual-gauge tracks are by far the most common configuration, but triple-gauge tracks have been built in some situations. Background The rail gauge is the most fundamental specification of a railway. Rail tracks and wheelsets are built within engineering tolerances that allow optimum lateral movement of the wheelsets between the rails. Pairs of rails that become too wide or narrow in gauge will cause derailments, especially if in excess of normal gauge-widening on curves. Given the requirement for gauge to be within very tight limits, when the designed distance between the pair of wheels on a wheelset differs even slightly from that of others on a railway, track must be built to two specific gauges. That is achieved in a variety of ways: most commonly by adding a third rail, more rarely by adding another pair of rails; and rarer still, when three gauges are present, by four rails. Configurations Dual-gauge track can consist of three rails, sharing one "common" rail; or four rails, with the rails of the narrower gauge lying between those of the broader gauge. In the three-rail configuration, wear and tear of the common rail is greater than with the two other outer rails. In dual gauge lines, turnouts (railroad switches) are more complex than in single-gauge track, and trains must be safely signalled on both of the gauges. Track circuits and mechanical interlocking must also operate on both gauges. Multi-gauge track is very often associated with a break-of-gauge station, where rail vehicles or vehicle contents are transferred from one gauge to another. A break of gauge causes delay and increases congestion, especially on single-track lines. Essentially, two trains are