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 infrastructure although in some places, for example the Central line east of St Paul's station, tunnels are above each other. The running tunnels are on the right on the Victoria line between Warren Street and King's Cross St. Pancras, allowing cross-platform interchange with the Northern line between northbound and southbound trains at Euston. Six of the 32 London boroughs are not served by the Underground. All of these are south of the River Thames: Bexley, Bromley, Croydon, Kingston, Lewisham and Sutton. The borough of Lewisham used to be served by the Underground on the East London line – now part of