The standard gauge used by most of the world’s railways is 1,435 millimetres.
Standard gauge measures the distance between the inside faces of the two rail heads. It is also expressed as 4 feet 8½ inches and is sometimes called Stephenson gauge, international gauge, or UIC gauge. The measurement became dominant partly because influential early British railways adopted it and exported their engineering practices, equipment, and track standards abroad.
A major advantage of a common gauge is interoperability: locomotives, carriages, and freight wagons can operate across connected networks without changing wheels or transferring cargo. Standard gauge is widespread in much of Europe, North America, China, and many high-speed railway systems.
It is not universal. India uses broad gauge on most of its main network, Russia and several neighbouring countries use 1,520 mm or related gauges, and Japan’s conventional railways mainly use 1,067 mm, while its Shinkansen lines use standard gauge. Gauge is also distinct from loading gauge, which describes the maximum vehicle size and clearance permitted around a track.