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.
You're not wrong:
However, that [regeneraative braking] can only work where trains are accelerating and braking at the same time, on the same electricity sub-station loop.
1) modern railway is fully IGBT powered. In this case it is trivial to inject current.
2) with DC current you need a substation capable of converting AC to DC (easy: bridge rectifier) but also DC to AC (to given tolerances) which is much more cumbersome.
