The London Underground (or simply the Underground or the Tube) is a public rapid transit system serving a large part of Greater London and parts of Buckinghamshire, Hertfordshire and Essex. The network is considered the oldest rapid transit system, incorporating 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 2014/15 carried 1.305 billion passengers, making the Underground the world's 11th busiest metro system.