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 Europe 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. 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).

A medium-capacity system (MCS), also known as light rapid transit or light metro, is a rail transport system with a capacity greater than light rail, but less than typical heavy-rail rapid transit. MCS's trains are usually 1–4 cars. Most medium-capacity rail systems are automated or use light rail type vehicles.