Rail gauge is the distance from the inside of one rail on a railroad track to the inside of the other. Most countries use a standard gauge of 1,435 mm (4 ft, 8 1/2 in). Wider gauges are called broad gauge, smaller gauges are called narrow gauge. Broad gauge may be used where the track needs to have heavy things moving on it. Narrow gauge is used to build railroads for less money, because the smaller track costs less. Places where different gauges meet are called break of gauge.
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Narrow gauge work train in an East Side Access cavern that will eventually house a standard gauge station for the Long Island Rail Road.
Rail gauge Facts for Kids. Kiddle Encyclopedia.