> How many watts of extra energy would be required to accelerate each kilogram of batteries on the train?
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.
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_.