It was rush hour. The northbound 6 train was packed, pulling into 33rd Street. The doors opened, and the crowds shoved in and out. Someone shouted and another snapped back; two men were in a shoving match at the wall. The bigger one wound up and punched hard; the second and collapsed on the bench.

“Stay there!” The attacker got onto our train. The doors closed behind him. He looked around at everyone. No one looked back. It was quiet in the car, silent except for the train on the tracks and the man’s heavy breathing. I was supposed to do something. I knew that. The idea of explaining to the man that what he did was wrong popped into my head. He looked my way and I looked past him. The train swayed through the tunnel. The man was given a lot of space as we pulled into Grand Central as he went to the doors. I considered following and pointing him out to a policeman. He left. I did nothing. Nobody did. The train doors closed and he was gone.

A couple of years after that, I was on the northbound 4 train just after midnight; we pulled out of Brooklyn Bridge. A man yelled out to the riders. “Watch this!” A boy, maybe 10 years old, assumedly his son, did a remarkable dance, spinning on the ground, flipping head over heels. He was very good. The father passed around his hat. I stared back at him. “This city has child labor laws, you know.”

“What’s that?” He had bad eyes, dark and small.

“You’re not allowed to do that.”

The train had arrived at 14th Street. The man turned away and then suddenly kicked at me sideways, right on my ass, hard, and walked out, his boy behind him. Nobody seemed to have noticed any of it.

There is a new Metro Transit Authority announcement these days: Stay Back from the Platform Edge. Recently there have been a couple of highly publicized incidents of people being pushed to their deaths on the tracks. I’ve always tended to be an edge-stander, but now I check behind me and stay a few feet back.