I had never seen one of these before today. They’re great. Way better than reaching for a single pole in a crowded car and wrapping your hand around some stranger’s fingers. And If you have the whole thing to yourself you can hook your arm through it.
Realistically, the only way a commenter train is going to change direction or decelerate that violently is if something derails it or slams into it.
Trains like this don’t slow down very quickly, even if you pull the emergency break. That’s why people are allowed to stand, walk, etc.