These “kids” are always primed to go off. The guy might have accidently bumped into one of them (they usually sprawl their legs out into the aisle), for example, and not apologized in a way they thought was sufficient. Or maybe he even asked if they could move; it looked like he had been trying to get off the train or at any rate was standing near the door.
They’re always angry. They get up in the morning angry, they’re angry all day, and it takes nothing to set them off.
I'm rather large (particularly compared to the Japanese) but not particularly intimidating unless I have to be.
On one morning commute, we had one of the 20is bosozoku (motorcycle gang) types seated in a crowded subway car and, as it turned a corner and shifted, a middle aged guy accidentally stepped on his outstretched feet.
Even though the guy quickly apologized, the seated bosozoku begin to get really belligerent. The Japanese in the area started moving as far away as possible from the brat as it looked like a pending fight. I don't like fights but I don't like bullies either. So as they moved away, I worked myself toward him. Just as he was about to get up and take a swing at the guy, I firmly placed my size 12 foot on his size 8 and said (in Japanese) "He apologized. Let it go or I'll get you off at the next stop to visit with subway security!"
He quickly sank back into his seat and behaved himself until my stop four or five stations later. I have seen subway security work in far less serious incidents than this (such as a belligerent drunk). The usually have four or five fairly burly cops with truncheons board the car a few stops later and quickly take the perp off the train. What happens after that, I'm not sure. But I do know that the Japanese do not like their train schedules disrupted even by the less than 60 seconds or so which it takes to remove one of these perps.
I am in the No 4 subway ALL THE TIME. Trust me i know