Park, where the two dissimilar systems meet, just a gap longer than one coach of a Bakerloo line train at the entry to (and exit from) the Bakerloo, which operates with a nominal -210V on the fourth rail and +420V on the third rail. There is no bridging of the incompatible systems as trains pass from one to the other since, like all UK electric trains intended to run extensively in tunnels, there is no continuity of traction power circuits between vehicles of the train. A similar arrangement applies between Putney Bridge and Wimbledon, where the District line runs over tracks owned by Network Rail, which is also used by South Western Railway, though normally only for stock movements. Northern City Line The Northern City Line connects the East Coast Main Line to Moorgate. It was isolated by the abandonment of the 1930s New Works Programme (and the development of the Metropolitan Green Belt). Tube services were truncated at its northern end by the Victoria line in 1964 at Drayton Park. The remainder was handed over to British Rail in 1975 in conjunction with the suburban electrification of the East Coast Main Line. The line uses third-rail DC electrification between Moorgate and Drayton Park, where trains switch to 25 kV AC overhead. 630 V DC, fourth rail (top contact) London Underground The London Underground is a large metro system operating across Greater London and beyond, commonly known as "the Tube". Its 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 +420VDC and the inner fourth rail is the negative return at –210VDC, giving a supply voltage of 630VDC. 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