Circle line (London Underground) with the other London Underground sub-surface lines: the Hammersmith & City line from Hammersmith to just before Aldgate; the Metropolitan line from Baker Street to Aldgate; and the District line from Tower Hill station to Edgware Road station. The line is electrified with a four-rail DC system: a central conductor rail is energised at and a rail outside the running rail at , giving a potential difference of . The running rails are not electrified. Much of the double track railway from the Hammersmith terminus to Westbourne Park station is on a high brick viaduct. East of Westbourne Park the
London Underground 1986 Stock consultation results show that the blue prototype was the favoured, and provided the core design basis for the 1992 Stock that was built for London Underground's Central line and Network SouthEast's Waterloo & City line. London Underground 1986 Stock The London Underground 1986 Tube Stock consisted of three prototype electric multiple units built for London Underground that led to the development of the 1992 Stock. In 1984, London Transport ordered three different prototype trains to test new materials, construction methods and seating layouts. Two were built by Metro-Cammell at its Washwood Heath factory, and the third by British Rail Engineering