The Metropolitan line is a London Underground service that connects Aldgate in the City of London, the capital's financial heart, with Amersham and Chesham in Buckinghamshire, with branches to Watford and Uxbridge. Coloured magenta on the tube map, the line serves 34 stations in 41.4 miles (66.7 km), and is the oldest rapid transit line in the world. The section between Aldgate and Baker Street is shared with the Circle and Hammersmith & City lines, between Rayners Lane and Uxbridge with the Piccadilly line, and between Harrow-on-the-Hill and Amersham with Chiltern Railways. Just under 67 million passenger journeys were made on the line in 2011/12.
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 Verney Junction in Buckinghamshire, more than 50 miles (80 km) from Baker Street. From the end of the 19th century, the railway shared tracks with the Great Central Railway route out of Marylebone. 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 toAylesbury. 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 S Stock trains entered service between 2010 and 2012.