Gauge when used in reference to a prototype railway is a measurement of the distance between the rails that form the running surface of the track.

Here in Australia, there are three gauges which have been commonly used since the first railways were constructed in Sydney and Melbourne in the mid 1800's:

• - Narrow gauge is the gauge which is employed heavily in Queensland, and was used to some degree in South Australia prior to the construction of the standard gauge lines that cross from east to west. It is still predominant also in Western Australia and Tasmania, and it was the main gauge used in WA until the Trans-Australian Railway was opened in 1970.

  The width of narrow gauge track is 1067 mm, which is 3'6" in the old lingo.

• - Standard gauge is the gauge which is employed mostly here in New South Wales, and is the universal gauge used throughout almost all of the North American continent also. Standard gauge now extends to Perth in WA thanks to the TAR, and also both to and from Melbourne thanks to the One Nation gauge standardisation programme in the 1990's which was an initiative (albeit very politically short-sighted) of the then Federal Labour government.

  The width of standard gauge track is 1435 mm, which is 4'8.5" in the old lingo.

• - Broad gauge is the gauge which is employed mostly in Victoria and parts of South Australia also. It is commonly referred to as Irish gauge.

  The width of broad gauge track is 1600 mm, which is 5'3" in the old lingo.

A number of other gauges have (or still are) employed for railways in mines, sugar cane plantations and mills, and above-ground mineral operations.