The London Underground (also known as the Tube or simply the Underground) is a public rapid transit system serving a large part of Greater London and parts of the home counties of Buckinghamshire, Hertfordshire and Essex.

The system serves 270 stations and has 402 kilometres (250 mi) of track, 52% of which is above ground. The network is considered the oldest rapid transit system, incorporating the world's first underground railway, the Metropolitan Railway, which opened in 1863 and is now part of the Circle, Hammersmith & City and Metropolitan lines