The Glasgow Subway is an underground metro line in Glasgow, Scotland. Opened on 14 December 1896, it is the third-oldest underground metro system in the world after the London Underground and the Budapest Metro. It is the only heavy rail underground metro system in the British Isles outside London, and also the only one outside London which operates completely underground. It is also one of the very few railways in the world with a track running gauge of 4 ft (1,219 mm).