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