Look, you really shouldn't hit on women in the subway. It's a cylindrical, magnified, fast-moving faux-pas; where even the slightest wrong move could make her feel uncomfortable and unsafe. So, if you feel like it's a bad idea, don't don't do it.
However, if you're facing a do-or-die, "I gotta see about a girl" situation, then here's what you should do to inflict the least possible amount of creepiness into the air.
We enlisted legendary body language and communication expert, Dr. Lillian Glass (no relation) to guide us and ensure you don't ruin a nice woman's morning commute and your subway pickup doesn't get... derailed.
For the love of God, don't be a creepy idiot
Unlike the traditional setting of a bar or club, a train offers little chance for a woman to escape from a rogue romantic aggressor. That aggressor becomes you if you mess up and act like the kind of guys you see on the front page of Gothamist.
You definitely do not want to make her uncomfortable or awkward with intensely sexual or suggestive statements -- actually, you shouldn't do that to any woman anywhere.