Yes indeed the London underground does use regenerative braking on many of their lines. The cool thing about a direct rail power system is that the voltage generated from trains that are braking can be fed back into the power rail to instantaneously power other trains on the same line that are accelerating. No need to carry the extra weight of a battery or flywheel. And like you say, it helps keep the tunnels cooler.

It has to be the same segment, which is a bit of a pain to manage. The amount of power being fed in is getting close to megawatts.