But even leaving aside that, if you're right next to the door when the alarm sounds, there generally is time for an able-bodied person to enter/exit before the door actually closes, the problem with having the same sound for auto-closing and train-about-to-leave is that it can easily cause people on the platform to panic and incorrectly think they are about to miss the train when in fact all it is is that the door nearest them is auto-closing and they still have ample time to walk along the platform and select which door they want to board from. Yes, as you say, they will see a few seconds later that they can still board, but that doesn't change that they've already had a moment of panic - which would be entirely avoidable with a better designed system.