There are two distinct types of train on the London Underground: the 
larger surface stock operating in the original cut-and-cover 
construction twin-track tunnels on the Circle, District, East London, 
Hammersmith & City and Metropolitan Lines; and the smaller tube stock 
running in the single-track deep-level tunnels on the Bakerloo, 
Central, Jubilee, Northern, Piccadilly, Victoria and Waterloo & City 
Lines. Tube gauge cars are 750mm (2 ft 6 in) lower in height than 
surface stock, with a floor height above rail level of 610mm (2 ft) 
compared with 980mm (3 ft 3 in) on surface stock.  

Almost all Underground trains are made up of two units consisting of 
three or four cars permanently coupled together. Common to all the 
lines on the London Underground is both the gauge of the tracks, which 
is the standard 1,432mm (4 ft 8.5 in), and the electric supply system, 
630 volts dc (direct current) with separate positive and negative 
rails.   

Bakerloo                  Tube      Refurbished 1972 Mk II Tube Stock
Central                   Tube      1992 Tube Stock
Circle                    Surface   Refurbished C69/77 stock
District (Main Service)   Surface   D78 stock
District (Wimbleware)     Surface   Refurbished C69/77 stock
East London               Surface   Refurbished A60/62 stock
Hammersmith & City        Surface   Refurbished C69/77 stock
Jubilee                   Tube      1996 Tube Stock
Metropolitan              Surface   Refurbished A60/62 stock
Northern                  Tube      1959/62 & 1995 Tube Stock
Piccadilly                Tube      Refurbished 1973 Tube Stock
Victoria                  Tube      Refurbished 1967 Tube Stock [1]
Waterloo & City           Tube      1992 Tube Stock

[1] Also includes some converted 1972 Mk II units.