I think the worst is feeling sick and like you're going to throw up in a very crowded subway car. The rocking and sudden jerking motion of the train, the sudden change in speeds, the proximity of people around you, the lack of air, the sudden heat, the high potential you won't find a place to sit down, and the potential smells will just wreck you completely. And if you do puke, you've got nowhere to do it but in a place that's probably going to hit someone.

Thankfully, I've never had this happen to me and I sincerely hope it never does.