630 V DC.2C fourth rail .28top contact.29 Railway electrification in Great Britain

the tube s fourth-rail system



london underground track, showing third , fourth rails beside , between running rails



london underground (lu)

the london underground large metro system operating across london , beyond, commonly known tube . 408-kilometre (254 mi) made of 11 lines; electrification began during 1890s. largely unified between 1900 , 1910 , nationalised in 1933, becoming railway component of london transport (lt). major expansion programme (the new works ) launched, in lt took on several urban branches of mainline railways.

the underground in north london; expansion south london limited geology unfavourable tunnelling , extensive main-line network, of being electrified (see southern electric ). underground uses relatively uncommon 4 rail system of electrification. 2 standard gauge rails running rails; outer third rail carries positive current @ +420 v dc , inner fourth rail negative return @ –210 v dc, giving supply voltage of 630 v dc. chief advantage of fourth-rail system that, in tunnels metallic (usually cast-iron) lining, return traction current not leak lining causing electrolytic corrosion there or in neighbouring utility mains. 2 running rails available exclusively track circuits.

the surface sections use fourth rail solely operational consistency; system shares track network rail in several places. track shared 750 v third-rail stock, central rail bonded running rails , outside rail electrified @ 660 v. allows both types of train operate satisfactorily. suburban network of london & north western railway (lnwr) electrified in co-operation underground, during 1970s british rail introduced third-rail emus , sections of lnwr suburban network not used underground had fourth rail removed (see london , north western railway , above).

the underground has carried out studies consider raising voltage above present 630 v nominal. new equipment @ substations allow future increase standard 750 v nominal. in addition, electrical equipment of new trains based on use of 750 v rated equipment. so, whilst new equipment being designed 750 v operation, no decision increase voltage has yet been made public underground.