You are (usually) a middle-aged businessman with a coat, a bag, and - if it's raining - an umbrella. How long on average does it take you to deal with these - whilst blocking the aisle - and sit down? If the answer is "longer than it takes for me to do half the Hard Sudoku puzzle in today's Metro" you are doing something very wrong, and doing it surprisingly often
It's usually that sort of person who stands in the aisle while faffing around with their stuff, with no consideration that their backside is practically in the face of the person seated in the opposite aisle. And then swings their coat around oblivious to the swiping effect on anyone within range.
My pet hate, one I always have to at least try to move seats from, is people who stink of smoke sitting nearby. It's obviously on their clothes, but it turns my stomach.
Not an "on the train" one, but people who don't observe the "stand to the right" convention on escalators, and either block both sides by standing in the middle, or a couple who simply have to ride up or down them side by side. Actually that is the one that really gets my goat, I have been known to insinuate myself past the culprit/s and remark "you should stand on the right, keep the left clear". Mostly I just get blank looks.