The LU is a rapid transit system serving a large part of Greater London and neighbouring areas of Essex, Hertfordshire and Buckinghamshire in the UK.
Despite the name, it is not the only underground railway to have been built in London - there was also the now defunct London Post Office Railway, Kingsway Tramway Subway and Tower Subway. There are localised railways in use today - the Docklands Light Railway and the Tramlink.
With its first section opening in 1863, it was the first underground railway system in the world. In 1890 it became the first to operate electric trains. Despite the name, about 55% of the network is above ground. It is usually referred to officially as 'the Underground' and colloquially as the Tube, although the latter term originally applied only to the deep-level bored lines, along which run slightly lower, narrower trains along standard-gauge track, to distinguish them from the sub-surface "cut and cover" lines that were built first. More recently this distinction has been lost and the whole system is now referred to as The Tube, even in recent years by its operator in official publicity.
Since 2003 LU has been a wholly owned subsidiary of TfL, the statutory corporation responsible for most aspects of the transport system in Greater London, which is run by a board and a commissioner appointed by the Mayor of London.
The Underground has 270 stations and around 400 kilometres (250 miles) of track, making it the second longest metro system in the world by route length after the Shanghai Metro. It also has one of the highest number of stations. In 2009, more than one billion passenger journeys were recorded, making it the third busiest metro system in Europe after Paris and Moscow.
The tube map, with its schematic non-geographical layout and colour-coded lines, is considered a design classic, and many other transport maps worldwide have been influenced by it.