This would require barriers on all the platforms, with doors aligned to the train doors. That in turn would require standardizing train cars sufficiently that the doors of all trains opened at the appropriate places.

This isn't impossible: the London Underground Jubilee line has barriers. But it's nontrivial and not cheap. Most of the Underground lines don't have it. Nor does the Montreal metro. (As far as I know, neither do Toronto, Paris, or Hong Kong, but my information on each of those is several years old.)