The Circle line is a London Underground service in a spiralling shape, running from Hammersmith to Edgware Road and then looping once around central London back to Edgware Road. The railway is below ground in the central section and on the loop east of Paddingdon, and, like London's deep-level tube railways, the railway tunnels are just below the surface and of similar size to those on British main lines. Coloured yellow on the tube map, the circle line serves 36 stations, including most of London's main line railway termini. Most of the route and all of the stations are shared with the District, Hammersmith & City and Metropolitan lines. On the Circle line and the Hammersmith & City line combined, over 114 million passenger journeys are made each year.
The first section opened in 1863 when the Metropolitan Railway opened the world's first underground line between Paddington and Farringdon with wooden carriages and steam locomotives. The same year a select committee report recommended an 'inner circle' of railway lines connecting the London railway termini, and the Metropolitan District Railway (commonly known as the District Railway) was formed to build the southern portion of the line. Due to conflict between the two companies it was October 1884 before the inner circle was completed. The line was electrified in 1905, and in July 1933 the two companies were amalgamated into the London Passenger Transport Board. In 1949 the Circle line appeared as a separate line for the first time on the Tube map. In December 2009 the closed loop around the centre of London on the north side of the River Thames was broken at Edgware Road and extended west to become a spiral serving Hammersmith.
The signalling system is being upgraded and the C Stock trains have been recently been replaced by new 7-car S Stock trains, in a programme scheduled for completion by 2015.