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 battery-electric locomotives ground. A number of the machines were fitted with Automatic Train Operation (ATO) equipment to enable them to work on the newly opened Victoria line. Subsequently, some have been fitted with Automatic Train Protection (ATP) equipment, compatible with the system used on the Central line, and some with ATP equipment which enables them to work with the replacement Distance-to-Go Radio (DTG-R) system on the upgraded Victoria line. The first two battery locomotives supplied for the London Underground were manufactured by Hurst Nelson and Co, who were based in Motherwell. They were delivered in August 1905, and were used during the