The Glasgow Subway is an underground light metro system in Glasgow, Scotland. Opened on 14 December 1896, it is the third-oldest underground rail transit system in the world after the London Underground and the Budapest Metro. It is also one of the very few railways in the world with a track running gauge of 4 ft (1,219 mm). Originally a cable railway, the subway was later electrified, but the double-track circular line was never expanded. The line was originally known as the Glasgow District Subway, and was thus the first mass transit system to be known as a "subway"; it was later renamed Glasgow Subway Railway. In 1936 it was renamed the Glasgow Underground. Despite this rebranding, many Glaswegians continued to refer to the network as "the Subway". In 2003, the name "Subway" was officially readopted by its operator, the Strathclyde Partnership for Transport (SPT).
The system is not the oldest underground railway in Glasgow: that distinction belongs to a three-mile (five-kilometre) section of the Glasgow City and District Railway opened in 1886, now part of the North Clyde Line of the suburban railway network, which runs in a tunnel under the city centre between High Street and west of Charing Cross. Another major section of underground suburban railway line in Glasgow is the Argyle Line, which was formerly part of the Glasgow Central Railway.