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 stations that are listed buildings which he described as his "brick boxes with concrete lids", many of which are listed, five at Grade II*. Holden's design for the Underground's headquarters building at 55 Broadway including avant-garde sculptures by Jacob Epstein, Eric Gill and Henry Moore, incorporates St James Park station and is listed Grade I. London Underground stations that are listed buildings The London Underground is a metro system serving a large part of Greater London and parts of Buckinghamshire, Hertfordshire and Essex. Seventy-one of the 270 London Underground stations use buildings that are on the Statutory List of Buildings of Special Architectural or Historic