The Hammersmith & City line of the London Underground runs between Hammersmith and Barking. Coloured salmon pink on the tube map, it serves 29 stations in 15.8 miles (25.5 km). It is underground in the central section between Paddington and Bow Road; between Farringdon and Aldgate East it skirts the City of London, the capital's financial heart. The tunnels are just below the surface and are a similar size to those on British main lines. Most of the track and all stations are shared with the District, Circle or Metropolitan lines, the other parts of London Underground's sub-surface railway, and over 114 million passenger journeys are made each year on this line and the Circle line.
In 1863 the Metropolitan Railway began the world's first underground railway service between Paddington and Farringdon with wooden carriages and steam locomotives. The following year a railway west from Paddington to Hammersmith opened and this soon became operated and owned jointly by the Metropolitan and Great Western railways. The line was extended to the east in stages, reaching theEast London Railway in 1884. The line was electrified in 1906, and in 1936, after the Metropolitan Railway had been absorbed by London Passenger Transport Board, some Hammersmith & City trains were extended over the former District Railway line to Barking. The Hammersmith & City route was shown on the tube map as part of the Metropolitan line until 1990, when it appeared as a separate line