I know some cities (Toronto? Boston?) have subways where there are platforms on both sides of the train, so the doors can be opened on one side for passengers to exit before the doors on the other side of the train open to let passengers enter. The (relatively) new AirTrain to JFK airport in Manhattan has glassed-in platforms with sliding doors. New York's subways are old, and I could imagine any kind of retrofitting would be a challenge and a huge expense that most people would argue would be better applied to building more lines to reduce crowding.