Some 60% of the world's railways use the standard gauge of 1.4 m (4.7 ft). Gauges vary by country and sometimes within countries. The choice of gauge during initial construction was mainly in response to local conditions and the intent of the builder.

Standard gauge (1,435 mm or 4 ft 8½ in) is the most widely used railway track gauge in the world, utilized by approximately 60% of all global railway lines. It is the primary standard in over 100 countries and territories, including the entirety of North America (USA, Canada, Mexico), the majority of Europe (except for the Broad Gauge in Russia, Spain, and Ireland), and China. It is also the international standard for high-speed rail; even countries that use different gauges for their traditional networks, such as Japan with its "Cape Gauge" (1,067 mm) and Spain with its "Iberian Gauge" (1,668 mm), have built their high-speed lines (like the Shinkansen and AVE) using standard gauge to allow for easier technological integration. The gauge originated in Britain with George Stephenson's early locomotives and was adopted internationally because it provided a stable balance between speed and load-carrying capacity for 19th-century steam engines.