<Ul> <Li> </Li> <Li> </Li> <Li> </Li> </Ul> <P> The London Underground (also known simply as the Underground, or by its nickname the Tube) is a public rapid transit system serving London and some parts of the adjacent counties of Buckinghamshire, Essex and Hertfordshire in the United Kingdom . </P> <P> The world's first underground railway, the Metropolitan Railway, which opened in 1863, is now part of the Circle, Hammersmith & City and Metropolitan lines; the first line to operate underground electric traction trains, the City & South London Railway in 1890, is now part of the Northern line . The network has expanded to 11 lines, and in 2016--17 carried 1.379 billion passengers, making it the world's 11th busiest metro system . The 11 lines collectively handle approximately 4.8 million passengers a day . </P> <P> The system's first tunnels were built just below the surface, using the cut - and - cover method; later, smaller, roughly circular tunnels--which gave rise to its nickname, the Tube--were dug through at a deeper level . The system has 270 stations and 250 miles (400 km) of track . Despite its name, only 45% of the system is actually underground in tunnels, with much of the network in the outer environs of London being on the surface . In addition, the Underground does not cover most southern parts of Greater London, with less than 10% of the stations located south of the River Thames . </P>

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