Broad-gauge railways use a track gauge (distance between the rails) greater than the standard gauge of 1,435 mm (4 ft 8 ¹⁄₂ in).

In Britain the Great Western Railway, designed by Isambard Kingdom Brunel, pioneered broad gauge from 1838 with a gauge of 7 ft 0 ¹⁄₄ in (2,140 mm), and retained this gauge until 1892. A number of harbours also used railways of this gauge for construction and maintenance. These included Portland Harbour and Holyhead Breakwater, which used a locomotive for working sidings. As it was not connected to the national network, this broad-gauge operation continued until the locomotive wore out in 1913.

It became apparent that standardization on a single gauge throughout a rail transport system was advantageous. Rolling stock did not need to match the gauge exactly; a difference of a few millimeters could be coped with, so that interoperability on systems with gauges only slightly different was possible.

While the parliament of the United Kingdom of Great Britain and Ireland was initially prepared to authorise lines built to the broad gauge of 7 ft 0 ¹⁄₄ in (2,140 mm), it was eventually rejected by the Gauge Commission in favour of all railways in the British Isles being built to standard gauge of 1,435 mm (4 ft 8 ¹⁄₂ in), this being the gauge with the highest route-mileage. Ireland, using the same criteria, was allocated a different standard gauge, Irish gauge. Broad-gauge lines in Britain were gradually converted to dual gauge or standard gauge from 1864, and finally the last of Brunel's broad gauge was converted over a single weekend in 1892.