Minimum-gauge railways have a gauge of most commonly 15 in (381 mm),^[1] 400 mm (15+3⁄4 in), 16 in (406 mm), 18 in (457 mm), 19 in (483 mm), 500 mm (19+3⁄4 in) or 20 in (508 mm). The notion of minimum-gauge railways was originally developed by estate railways^[1] and the French company of Decauville for light railways, trench railways, mining, and farming applications.^[2]

The term was originally conceived by Sir Arthur Percival Heywood, who used it in 1874 to describe the principle behind his Duffield Bank Railway, specifically its 15 in (381 mm) gauge, distinguishing it from a "narrow gauge" railway. Having previously built a small railway of 9 in (229 mm) gauge, he settled on 15 in (381 mm) as the minimum that he felt was practical.^[1] The original text of Heywood's article defining minimum gauge railways is available online.^[3]

In general, minimum-gauge railways maximize their loading gauge, where the dimension of the equipment is made as large as possible with respect to the track gauge while still providing enough stability to keep it from tipping over. Standard gauge railways have vehicles that are approximately twice, and in some cases nearly three times, the track gauge in width, but with minimum gauge railways this can be as much as four times the width of the track as in some of the Sugar Cane Railways of Australia. Minimum-gauge railways allowed for ease of mobility on battlefields, mines, and other restricted environments.

A number of 18 in (457 mm) gauge railways were built in Britain to serve ammunition depots and other military facilities, particularly during the First World War.

In South Australia the Semaphore to Fort Glanville Conservation Park includes a steam engine service that runs on an 18 in (457 mm) track.

In France, Decauville produced a range of portable track railways running on 400 mm (15+3⁄4 in) and 500 mm (19+3⁄4 in) tracks, most commonly in restricted environments such as underground mine railways, parks and farms.^[2]

During World War II, it was proposed to expedite the Yunnan–Burma Railway using 400 mm (15+3⁄4 in) gauge, since such a small gauge can have the tightest of curves in difficult terrain.^[4]

This section possibly contains original research. (May 2020)

The major distinction between a miniature railway (US: 'riding railroad' or 'grand scale railroad') and a minimum-gauge railway is that miniature lines use models of full-sized prototypes. There are miniature railways that run on gauges as wide as 2 ft (610 mm), for example the Wicksteed Park Railway. There are also rideable miniature railways running on extremely narrow tracks as small as 10+1⁄4 in (260 mm) gauge, for example the Rudyard Lake Steam Railway. Around the world there are also several rideable miniature railways open to the public using even narrower gauges, such as 7+1⁄4 in (184 mm) and 7+1⁄2 in (190.5 mm).

Generally minimum-gauge railways have a working function as estate railways, or industrial railways, or providers of public transport links; although most have a distinct function in relation to tourism, and depend upon tourism for the revenue to support their working function.