I don't go by train often, but when I'm in the bus I always have my bag in the seat next to me, because it's annoying to have it between your feet since that limits your feet positions. Unless it's certain that someone will sit next to me due to the influx of people I just let my bag sit there. I always sit next to the window though, so all they have to do is ask me to remove my bag, most of the times I get it without them just asking. You could find this rude, but I swear to god, I'm not even big in size and when someone sits next to you it's sometimes even better to stand then to sit in an uncomfortable position you can't get out of and your armpits starting to sweat because you have no space. Now imagine a fat person sitting next to you and taking part of your seat. The seats are like reverse car seats, instead of them preventing you to slide out of your seat they make you slide out of it because it's downwards. The woman in this pic is still rude though, feet on the seats instead of on the ground or the stuff below the seat which they won't sit on, and having a bag on the opposites seat.

Damn, if only I put this much effort into school.