“The Tube” is the common nickname for London’s deep-level underground railway system.
The name comes from the roughly cylindrical tunnels used by deep-level railway lines. Their small circular profile was shaped by tunnelling methods and helped engineers pass beneath streets and buildings. Londoners commonly use “the Tube” even when referring broadly to the entire London Underground network.
The nickname is not the formal name of the transport system, which is London Underground. It also does not mean that every section of the network runs through a tube: many Underground routes travel in open cuttings, on embankments or at ground level.
Other cities use terms such as metro or subway, but “the Tube” is especially associated with London. The word appears in everyday travel announcements, maps, journalism and local conversation.