The railway track gauge measuring 1,435 millimetres and used by most railways worldwide is standard gauge.
Standard gauge is the distance between the inner faces of the two rails, measured at the defined gauge point. Its nominal width is 1,435 millimetres, equivalent to 4 feet 8½ inches. It is used across much of Europe, North America, China and many other railway systems, although “most railways worldwide” does not mean every country uses it.
The gauge became widespread through the influence of early British railway engineering, especially lines associated with George Stephenson and his successors. As networks expanded, compatibility with neighbouring railways became commercially and operationally valuable. Once a country or region had built a large network, changing gauge was usually expensive and disruptive.
Other gauges remain important. Russia uses a broad gauge of 1,520 millimetres, India commonly uses 1,676 millimetres, and Japan has extensive narrow-gauge lines. Gauge determines rail spacing, but it does not by itself specify train speed, loading capacity or the quality of a railway.