"In rail transport, track gauge or track gage is the spacing of the rails on a railway track and is measured between the inner faces of ...
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the load-bearing rails.
All vehicles on a rail 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.
In some places there is a distinction between the nominal gauge and the actual gauge, 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." - (en.wikipedia.org 30.06.2021)
