What track gauge is known as standard gauge on railways worldwide?

Standard gauge on railways worldwide is 1,435 millimetres, measured between the inner faces of the rails.

This gauge became widely established through the influence of George Stephenson’s early railway projects in Britain. The Liverpool and Manchester Railway and several other influential nineteenth-century lines used it, helping spread the measurement as rail networks expanded.

Standard gauge is not the most widespread gauge in every country, because broad-gauge and narrow-gauge systems remain important. India, Spain, Russia, and several other places have historically used different gauges on major routes. The term therefore describes a specific measurement, not a universal requirement.

Gauge should also be distinguished from loading gauge, which concerns the maximum size of trains and their clearance from structures. Track gauge measures the distance between rails, while loading gauge describes the space available around a vehicle.