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 GN&C Stock Heath. These plans were curtailed by World War II and the Northern City line (or "Highbury Branch") continued operating with 1938 Stock until 1975, when the line was transferred to British Rail. Since 1976 the line has been operated exclusively by British Rail Class 313 electric multiple units. London Underground GN&C Stock The GN & C Stock was operated on the Great Northern and City Railway, an underground railway line in London, England. It was opened between Moorgate and Finsbury Park on 14 February 1904. The original GN&C Stock operated on the line between 1904 and 1939. The line was