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 counties of Buckinghamshire, Hertfordshire and Essex.
The system serves 270 stations and has 402 kilometres (250 mi) of track, 55 per cent of which is actually above ground.
The network incorporates the world’s first underground railway, the Metropolitan Railway, opened in 1863 and 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 2011/12 it carried over 1 billion passengers. This year London Underground has celebrated 150 years of operations.