The railway system in Great Britain is the oldest in the world: the world's first locomotive-hauled public railway opened in 1825. Most of the railway track is managed by Network Rail, which in 2015 had a network of of standard-gauge lines, of which were electrified. These lines range from single to quadruple track or more. In addition, some cities have separate rail-based mass transit systems (including the extensive and historic London Underground). There are also several private railways (some of them narrow-gauge), which are primarily short tourist lines. The British railway network is connected with that of continental Europe by an undersea rail link, the Channel Tunnel, opened in 1994.