The London Underground (subway) system uses both 3rd AND 4th rails, which makes for many gaps in one or other conductor rail where trains run over pointwork. The supply is nominally 630V DC, with the 3rd rail (similar outside position to regular 3rd rail system) running at about +420V to ground and the 4th rail (centrally mounted between the running rails) at approx. -210V to ground. The supply is resistance-earthed rather than being solidly bonded, allowing trains to continue to run in the presence of a single ground fault.
Movements in service shops and like are made by connecting the DC supply directly to connectors provided on the trains to avoid having live conductor rails around.