The London Underground is a public rapid transit system serving London and some parts of the adjacent counties of Buckinghamshire, Essex and Hertfordshire in the United Kingdom.
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.
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