Track Gauge

Track gauge or rail gauge is the distance between the inner sides of the heads of the two load bearing rails that make up a single railway line. This is opposed to loading gauge which defines the maximum height and width for railway vehicles. Sixty percent of the world's railways use Stephenson's 4 ft 8½ in (1,435 mm) standard gauge. Wider gauges are called broad gauge; smaller gauges, narrow gauge. Break-of-gauge refers to the meeting of different gauges. Some stretches of track are dual gauge, with three or four rails, allowing trains of different gauges to share them. Gauge conversion can resolve break-of-gauge problems. Some electrified railways use non load bearing third rail and occasionally a fourth rail. These additional rails are positioned between or outside the “running rails” to feed and return electrical current; they do not define the rail gauge.

Technically, monorails have no gauge, since there is only one rail and a track gauge is a measure of the distance between two rails, but in practice the dimensions of the single rail, trough, or overhead support, including power supplies, govern whether one monorail line is compatible with another.

Gauge tolerances specify how much the actual gauge may vary from the nominal gauge. For example, the U.S. Federal Railroad Administration specifies that the actual gauge of track that is rated for a maximum of 60 mph (96.6 km/h) must be between 4 ft 8 in (1,422 mm) and 4 ft 9½ in (1,460 mm).

A track gauge is also the measuring device used to test whether rails are within the correct gauge.