I've seen it, but not nearly as often as OP seems to. Often people will stink of weed but not be actively smoking, but it's rare for me to see anyone lighting up on the train (and when it happens it's usually cigarettes). Most people just respect that it's fuckin rude to smoke in an enclosed space with people. I HAVE walked into a subway car where people were fully attempting to hotbox, like, twice in my 10yrs here.

What OP really needs to worry about is ear protection! The subway is so loud that commuting regularly without protection will do long-term damage to your hearing. (I don't worry too much about it myself, but OP seems in the mood to worry about things.)

I had a lovely experience last month with a young adult (she looked around 19/21) vaping right next to me in a packed train. I side-eyed and sighed loudly, she then proceeded to breathe smoke into my face twice.