The London Underground (also known as the Tube or simply the Underground) is a public metro 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, 55% of which is above ground. The network incorporates 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; and the first line to operate underground electric traction trains, the City & South London Railway in 1890, now part of the Northern line. The network has expanded to 11 lines, and in 2012/13 carried 1.23 billion passengers.
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