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. Coloured magenta (Pantone 235) on the tube map, the line is 41.4 miles (66.7 km) in length and serves 34 stations. The section between Aldgate and Baker Street is shared with the Circle and Hammersmith & City lines; that between Rayners Lane and Uxbridge with the Piccadilly line; and that between Harrow-on-the-Hill and Amersham with Chiltern Railways. The line is one of only two London Underground lines to cross the Greater London boundary, the other being the Central line.
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. wikipedia