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
Romford–Upminster line am and 8 pm on Sundays. The journey time from one terminus to the other is approximately nine minutes. During the construction of the railway in 1892, the geologist T. V. Holmes discovered a five-metre layer of boulder clay underlying the gravel and sand in a section just north of St. Andrew's Park. This is now the Hornchurch Cutting Site of Special Scientific Interest, which exhibits the southernmost limit of the Anglian ice sheet 450,000 years ago, the furthest south that any ice sheet reached. Romford–Upminster line The Romford–Upminster line is a railway branch line in Greater London that connects