By definition, the track spacing is given from centre to centre of a rail track. For an actual construction the distance is measured from the inside of a rail head to the matching one of the other track. As far as both tracks have the same gauge this is the same distance.

The distance between the two inner rails of a railroad track is known as the "Standard Gauge," and it is set at 4 feet 8.5 inches (1,435 mm) for approximately 60% of the world's railways. This specific measurement has a fascinating history; it was popularized by George Stephenson for the early British coal railways, which were built to match the width of existing horse-drawn wagons and the tramways that preceded them. While a popular urban legend suggests this width traces back to the wheel ruts of Roman chariots, the reality is more about the practical "turning radius" of early wooden carts. In 2026, while "Standard Gauge" dominates North America, Europe, and China, other regions use "Broad Gauge" (like India's 5 feet 6 inches) for better stability or "Narrow Gauge" (like 3 feet 6 inches in parts of Africa and Japan) to handle steep, winding mountain terrain more cheaply. The gauge is the most fundamental engineering constraint of a rail network, as it determines which locomotives and cars can travel between different countries and systems without needing expensive "bogey changes."