A narrow-gauge railway (narrow-gauge railroad in the US) is a railway with a track gauge narrower than standard 1,435 mm (4 ft 8+1⁄2 in). Most narrow-gauge railways are between 600 mm (1 ft 11+5⁄8 in) and 1,067 mm (3 ft 6 in).
Since narrow-gauge railways are usually built with tighter curves, smaller structure gauges, and lighter rails, they can be less costly to build, equip, and operate than standard- or broad-gauge railways (particularly in mountainous or difficult terrain). Lower-cost narrow-gauge railways are often used in mountainous terrain, where engineering savings can be substantial. Lower-cost narrow-gauge railways are often built to serve industries as well as sparsely populated communities where the traffic potential would not justify the cost of a standard- or broad-gauge line. Narrow-gauge railways have specialised use in mines and other environments where a small structure gauge necessitates a small loading gauge.
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