The Metropolitan line is a London Underground line that runs from Aldgate, in the City of London, to Amersham and Chesham in Buckinghamshire, with branches to Watford in Hertfordshire, and Uxbridge in the London Borough of Hillingdon formerly in Middlesex. Coloured magenta on the tube map, the line is 66.7km in length and serves 34 stations. Unlike the tube railways, the tunnels are just below the surface (cut and cover) and of similar size to those for mainline trains. Just under 67 million passenger journeys were made on the line in 2011/12. The line is distinguished as being more of a suburban railway; it is one of only two London Underground lines to cross the Greater London boundary (the other being the Central line) and it is also the only London Underground line with an express service - its longer distance stops allow it to achieve the system's highest train speeds of over 60 mph on some sections.
In 1863 the Metropolitan Railway began the world's first underground railway between Paddington and Farringdon Street with wooden carriages and steam locomotives, but its most important route became the line north into the Middlesex countryside, where it stimulated the development of new suburbs. Harrow was reached in 1880, and the line extended as far as in Buckinghamshire, more than 50mi from Baker Street. From the end of the 19th century, the railway shared tracks with the Great Central Railway route out of . The central London lines were electrified by 1907, but electric locomotives were exchanged for steam locomotives on trains heading north of Harrow. After the railway was absorbed by the London Passenger Transport Board in 1933 the line was cut back to . Steam trains ran until 1961, when the line was electrified to and services curtailed at Amersham. The Hammersmith & City line was shown on the tube map as part of the Metropolitan line until 1990, when it appeared as a separate line. The current S7 and S8 Stock trains entered service between 2010 and 2012.
The section between Aldgate and is shared with the Circle and Hammersmith & City sub-surface lines; that between and with the Piccadilly line; and that between Harrow-on-the-Hill and Amersham with Chiltern Railways. The track is mostly underground between Aldgate and, save from and Farringdon as well as the Baker Street platforms. Baker Street is the southbound terminus for some trains not continuing on to Aldgate. Most of the route is two tracks, except for the single-track Chesham branch and a four-track section between and that allow fast and semi-fast services to overtake 'all stations' trains. There are also four tracks between Wembley Park and Finchley Road, but the inner pair of tracks was transferred to the Bakerloo line in 1939, then to the Jubilee line in 1979, with services calling at all stations whilst the Metropolitan line passes them on the outer lines. The disused Metropolitan line platforms at and are still in place despite Metropolitan line services having been withdrawn in 1940.
The Watford branch is due to be diverted to by 2020, resulting in the closure of Watford station and the opening of two new stations. The extension will link up with the London Overground Watford DC Line and would provide an interchange with the West Coast Main Line.