Why don't they use batteries to buffer between consumption / regen and the upstream power?
see https://tfl.gov.uk/corporate/transparency/freedom-of-informa...
Seems not trivial to just put extra batteries and the circuitry for regen breaking charging in.
It may be better to install such batteries somewhere on each track circuit as fixed stations, that can vent their charging and dispersal heat away from passenger tubes, rather than mobile — assuming that onboard emergency power needs are already met.
That's not really the problem; the problem is there is _no space_. Ever been on a London Underground train? You certainly wouldn't want the interior getting _smaller_.
I'm sure they'd consider the worst-case scenario, and at present that's probably some serious arcing within a motor causing a fire. Cutting the track power (easily done by the driver) then makes that a fire that can be handled with water and/or foam.
Smoke from large batteries would be very hazardous.