Very good point. The doors do re-open 90.0% of the time. And the train will never take off if the doors in all cars are not fullu closed. However, there have been a few incidents(0.1%) in NYC when a train took off with someone's arm or some other part of the body sticking out of the door... Even, here in boring Boston, couple years ago some guy was stuck half-way in the door and the train(Red Line) took off. He was held by other 2-3 passengeers from falling out of the train until the next stop(That probably wouldn't happen in NYC :-)...He made it OK. The story made it to the Boston Globe...
-Mark