A narrow gauge railway (or railroad) is a railway that has a track gauge narrower than the 1,435 mm (4 ft 8 1⁄2 in) of standard gauge railways. Most existing narrow gauge railways have gauges of between 2 ft (610 mm) and 3 ft 6 in (1,067 mm).
Since narrow gauge railways are usually built with smaller radius curves and smaller structure gauges, they can be substantially cheaper to build, equip, and operate than standard gauge or broad gauge railways, particularly in mountainous or difficult terrain. The lower costs of narrow gauge railways mean they are often built to serve industries and communities where the traffic potential would not justify the cost of building a standard or broad gauge line.
Narrow gauge railways also have specialized use in mines and other environments where a very small structure gauge makes a very small loading gauge necessary.
On the other hand, standard gauge or broad gauge railways generally have a greater haulage capacity and allow greater speeds than narrow gauge systems.
Historically, many narrow gauge railways were built as part of specific industrial enterprises and were primarily industrial railways rather than general carriers. Some common uses for these industrial narrow gauge railways were mining, logging, construction, tunnelling, quarrying, and the conveying of agricultural products. Extensive narrow gauge networks were constructed in many parts of the world for these purposes.
For example, mountain logging operations in the 19th century often used narrow gauge railways to transport logs from mill sites to market. Trench railways on the western front in World War I were a short-lived military application. Significant sugarcane railways still operate in Cuba, Fiji, Java, the Philippines and in Queensland in Australia. Narrow gauge railway equipment remains in common use for the construction of tunnels.
Narrow gauge railways also have more general applications. Non-industrial narrow gauge mountain railways are or were common in the Rocky Mountains of the United States and the Pacific Cordillera of Canada, in Mexico, Switzerland, the former Yugoslavia, Greece, India, and Costa Rica. In South Africa the "Cape gauge" of 3 ft 6 in (1,067 mm) is the most common gauge. In India, the narrow gauge system is slowly being converted to broad gauge, although some of India's most famous railways, the Darjeeling Himalayan Railway and Kalka-Shimla Railway are both narrow gauge. All 1,000 mm (3 ft 3 3⁄8 in) (metre gauge) railways are being converted to 5 ft 6 in (1,676 mm) (broad gauge) under the Unigauge project.