Rail gauge

In rail transport, track gauge is the spacing of the rails on a railway track and is measured between the inner faces of the load-bearing rails.

All vehicles on a network must have running gear that is compatible with the track gauge, and in the earliest days of railways the selection of a proposed railway's gauge was a key issue.

As the dominant parameter determining interoperability, it is still frequently used as a descriptor of a route or network.

There is a distinction between the nominal gauge and actual gauge at some locality, due to divergence of track components from the nominal. Railway engineers use a device, like a caliper, to measure the actual gauge, and this device is also referred to as a track gauge.

The nominal track gauge is the distance between the inner faces of the rails. In current practice, it is specified at a certain distance below the rail head as the inner faces of the rail head (the gauge faces) are not necessarily vertical.

Rolling stock on the network must have running gear (wheelsets) that are compatible with the gauge, and therefore the gauge is a key parameter in determining interoperability, but there are many others—see below. In some cases in the earliest days of railways, the railway company saw itself as an infrastructure provider only, and independent hauliers provided wagons suited to the gauge. Colloquially the wagons might be referred to as "four-foot gauge wagons", say, if the track had a gauge of four feet. This nominal value does not equate to the flange spacing, as some freedom is allowed for.

An infrastructure manager might specify new or replacement track components at a slight variation from the nominal gauge for pragmatic reasons.

Track is defined in old Imperial units or in universally accepted metric units or SI units.

Imperial units were established in United Kingdom by The Weights and Measures of Act of 1824. The United States customary units for length did not agree with the Imperial system until 1959, when one International yard was defined as 0.9144 meters, i.e. 1 foot as 0.3048 meter and 1 inch as 25.4 mm.

...
Wikipedia