> indeed, the use of regenerative braking now converts about half the heat loss back into electricity. However, that can only work where trains are accelerating and braking at the same time, on the same electricity sub-station loop.

Besides that, Tube tracks often rise slightly at stations, so that trains get a small gravity assist to both stopping and starting off again. (Unfortunately it also means that hot air from the tunnels tends to collect there, but you can't win 'em all.)

Correct me if i'm wrong, but it seems that regenerative braking is a bit troublesome because it is a third rail direct current system: a "regular" AC system can simply feed power back through the transformers to the power grid, but this is not possible here, so power must be consumed by another train fed by the same rectifier.

The first generation of Munich subway trains (manufactured from 1967-1983) uses resistor banks for braking, so the energy from braking goes right into heat.

Only the later generation B and the new C generation can move brake energy back into the grid.