In article , Clive Coleman
writes
As far as I know all the Southern region stock has bus lines throughout
so any car with or without shoes could be motored whereas no
underground trains do.

Memory says that traction bus lines aren't permitted on underground
lines (as opposed to lines with the occasional tunnels).

The 313 dual voltage stock that runs to Moorgate over the old GNCR is
made up of three cars:
Driving motor (pickup shoes)
Pantograph trailer
Driving motor (pickup shoes)
In A.C. mode there's a through bus along the length of the train, fed in
the centre from the transformer. In D.C. mode (designed for the tubes)
the bus is disconnected and each motor car runs off its own pickup
shoes.

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