The US standard railroad gauge is 4 feet, 8.5 inches (Gauge means width between the two rails). The U.S. federal safety standards allow the standard gauge to vary from 4 ft 8 in (1,420 mm) to 4 ft 9 1/2 in (1,460 mm) for operation up to 60 mph (97 km/h).

The distance between the two parallel rails of a railroad track is called the Track Gauge (or simply "gauge"). Specifically, it is measured between the inside edges of the heads of the rails. The most widely used distance in the world—found in about 60% of global networks, including North America, China, and most of Europe—is Standard Gauge, which measures exactly 4 feet 8.5 inches (1,435 mm). Legend has it that this odd measurement originated from the wheel-span of Roman chariots, though historians point more directly to 19th-century British coal mine wagonways. Other variations include Broad Gauge (wider than 1,435mm), used in India and Russia for greater stability and load capacity, and Narrow Gauge (smaller than 1,435mm), used for mountainous terrain or specialized industrial lines. In 2026, maintaining a uniform "standard gauge" is critical for international rail "interoperability," allowing trains to cross borders without needing complex wheel-switching.