There are diesel locomotives that can use their 'starting' batteries to
move them selves - intended for workshop moves and short 'light' yard
moves - so the main diesel plant doesn't have to be started up (and
warmed up, etc, etc) just to move the engine to another maintenance road.

Just how much extra battery would an EMU have to carry (over what it's
already carrying to run the lights, control and safety systems) to move
the train a low speed to the next station platform to allow an orderly
evacuation ?.

Providing enough auxiliary power to run at service speed is just plain
stupid in an EMU, and even an small diesel aux is a maintenance hassle.
If nothing else the fuel will keep going off in the tanks as it doesn't
get used.

Providing enough battery to allow a set to limp to the next platform
might be some what useful and not that expensive in the grand scheme of
things.