incandescent electric lamps in the carriages worked, regardless of the polarity of the supply. The three-wire system was adopted because the initial system was fed directly from the dynamos in the surface power plant at the Stockwell end of the line. It was important to minimise the voltage drop as much as possible, bearing in mind the rather steep gradient on the approach to King William Street Station. 440 V DC, third rail London Post Office Railway Underground railway under London operated by the Post Office. Operated between 1927 and closure in 2003. Partially re-opened as a tourist attraction in 2017. See also British electric multiple units Campaign to Electrify Britain's Railways History of rail transport in Great Britain List of British electric locomotives References Further reading 525 V DC, third rail 630 V DC, fourth rail 650 V DC, third rail 750 V DC, third rail 1,500 V DC, overhead 6.6 kV 25 Hz AC, overhead 25 kV 50 Hz AC, overhead External links Citytransport.info's Electrification advocacy page Rail infrastructure in the United Kingdom Great Britain Train-related introductions in 1883