page_title: Railway electrification in Great Britain
section_title: 630 V DC, fourth rail (top contact)
hierarchical_section_title: Railway electrification in Great Britain / Existing systems - third and fourth rails / 630 V DC, fourth rail (top contact)
caption_attribution_description: Español: Ealing Common es la única estación donde las líneas District y Piccadilly utilizan los mismos andenes English: Unrefurbished London Underground District Line 'D'-stock electric multiple unit 7127 arrives into Ealing Common tube station on 22 October 2005.
context_page_description: Railway electrification in Great Britain began in the late 19th century. A range of voltages has been used, employing both overhead lines and conductor rails. The two most common systems are 25 kV AC, using overhead lines; and the 750 V DC third rail system used in southeast England and on Merseyrail. As at April 2019, 3,736 miles of the British rail network was electrified.
  According to Network Rail, as at 2003, 64% of the electrified network used the 25 kV AC overhead system, and 36% used the 660/750 V DC third-rail system.
  The electrified network is set to expand over coming years, as 25 kV electrification is extended to currently unelectrified lines such as the Midland Main Line, and lines in the North of England as part of the Northern Hub.
context_section_description: London Underground
  The London Underground is a large metro system operating across Greater London and beyond, commonly known as "the Tube". Its 408-kilometre (254 mi) is made up of 11 lines; electrification began during the 1890s. It was largely unified between 1900 and 1910 and nationalised in 1933, becoming the railway component of London Transport (LT). A major expansion programme (the "New Works") was launched, in which LT took over several urban branches of mainline railways.
  The Underground is mostly in north London; its expansion into south London was limited by geology unfavourable to tunnelling and by the extensive main-line network, much of which was being electrified (see "Southern Electric").
  The Underground uses a relatively uncommon four rail system of electrification. Two standard gauge rails are the running rails; the outer third rail carries positive current at +420 V DC and the inner fourth rail is the negative return at –210 V DC, giving a supply voltage of 630 V DC. The chief advantage of the fourth-rail system is that, in tunnels with a metallic (usually cast-iron) lining, the return traction current does not leak into the lining causing electrolytic corrosion there or in adjacent utility mains. It also means that the two running rails are available exclusively for track circuits.
  The surface sections use the fourth rail solely for operational consistency; the system shares track with Network Rail in several places. Where the track is shared with 750 V third-rail stock, the central rail is bonded to the running rails and the outside rail electrified at 660 V. This allows both types of train to operate satisfactorily. The suburban network of the London & North Western Railway (LNWR) was electrified in co-operation with the Underground, but during the 1970s British Rail introduced third-rail EMUs and the sections of the LNWR suburban network not used by the Underground had the fourth rail removed (see "London and North Western Railway", above).
  The Underground has carried out studies to consider raising the voltage above the present 630 V nominal. New equipment at their substations does allow for a future increase to a standard 750 V nominal. In addition, the electrical equipment of new trains are also based on the use of 750 V rated equipment. So, whilst new equipment is being designed to for 750 V operation, no decision to increase the voltage has yet been made public by the Underground.
caption: London Underground track, showing the third and fourth rails beside and between the running rails
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