There are about 6,535,342 things that you can do wrong on the train: eat, drink, listen to music, sneeze, fart, breathe, exist.
One thing that's considered rude is crossing your legs. It makes your foot stick out, and the dirt on your foot might touch someone else. And I get that. On a crowded train, it's important to make yourself as compact and inconspicuous as possible, because a lot of people need to inhabit that space.
Riding the subway home from work the other day, the train was half empty. Most of the seats were taken, but a few seats were vacant, and almost no one was standing, except me at the end of the car. A bench seat stretched out in front of me, 70% occupied with the usual suspects, including an older-middle aged lady (60s) dozing with her legs crossed. A middle aged man (50s) in casual clothes got on at the door closest to me and made his way down the aisle.
As he passed the snoozing lady, he looked down and gave her foot a swift, hard, kick. And kept walking. She woke up, startled and befuddled, and quickly righted herself and uncrossed her legs.
Now, some people get their panties all in a bunch about perceived rudeness. Some even take it upon themselves to correct others. This guy though, this prince among men, did all of the above... with his mobile phone glued to his ear.
What. A. Dick.
I glared at him, and he saw my glare. I hope he felt bad at least for getting caught, but he probably didn't.