track gauge
The specified distance between the rails of a railway.
track gauge: the distance between your rails
Track gauge is the perpendicular distance measured between the inner edges of the two rails that form a railway line. This measurement is taken at the top of the rail head and remains constant along the length of track. It determines compatibility: rolling stock (locomotives, cars, wagons) built for one gauge cannot operate on track of a different gauge without modification or additional wheels.
The most common gauge worldwide is standard gauge, which measures 1,435 millimetres (4 feet 8.5 inches). This dimension originated from early English railways and has become dominant in North America, Europe, and much of Asia. However, many regions use narrow gauges: 1,000 millimetres (metre gauge) is widespread in parts of Africa, Asia, and South America; 1,067 millimetres (3 feet 6 inches) runs through Japan, South Africa, and Australia. Broad gauges, wider than standard gauge, appear in Russia (1,520 mm) and India (1,676 mm). A few heritage and specialist railways use gauges as narrow as 600 millimetres or even smaller.
Why gauge matters in practice
Gauge is fundamental to infrastructure planning and procurement. It dictates the cost and type of rolling stock, the design of bridges and tunnels, and the operational compatibility of connected lines. Mismatched gauges between adjoining railway systems historically required trains to stop for wheel changes or cargo transloading, creating operational friction. Modern high-speed and heavy-freight railways maintain strict gauge tolerances because deviations of even 5 to 10 millimetres can cause derailment risks, increased maintenance costs, or operating restrictions.
Gauge is set and maintained during track construction and renewal. Gauge checking uses a gauge stick or digital caliper at regular intervals along the line. Worn or corroded rails, frost heave, and differential settlement of ballast all can shift gauge out of specification. Maintenance crews correct these deviations by adjusting rail fastenings or tamping ballast. Historic gauge changes are rare and enormously expensive, so once a railway is built to a given gauge, that choice typically endures for its operational life.