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 into the fire led to the resignation of senior management of both London Underground and London Regional Transport. To comply with new safety regulations issued as a result of the fire, and to combat graffiti, a train refurbishment project was launched in July 1991. In April 1994, the Waterloo & City Railway, by then owned by British Rail and known as the Waterloo & City line, was transferred to the London Underground. In 1999, the Jubilee line was extended from Green Park station through Docklands to Stratford station, resulting in the closure of the short section of tunnel between Green