I'm not going to sit next to someone if there are plenty of free seats where I don't need to sit next to anyone. But on a busy train then yes, I will seek out the person who is being all aggressive and difficult about not wanting anyone to sit next to them. Everyone else has to squash in, the passive-aggression really gets my goat. Why should the passive-aggression win, and why should the sensible considerate people always lose out?
I don't believe the stories of people throwing bags around the carriage though. I've only moved a bag once, gently into the overhead luggage rack: it was on a full-and-standing XC, the bag was on my reserved seat, and I didn't want to wake the person up (although I'm not entirely convinced he was asleep...)
Even though singling out someone in particular to pick on is, by it's very nature, passive aggressive.
Also, in case you hadn't noticed it's always the sensible, considerate, honest people who lost out. Case in point is insurance - I'm honest, so I end up paying ridiculous amounts compared to the casual dishonesty many people freely admit to engaging in. Similarly I'm considerate and follow the rules by buying a ticket appropriate to my needs, so I end up paying an absolute fortune for my train travel.
In return, it's only fair to expect other people to be considerate and polite, and ask (if they can) before sitting next to me, which in more cases than not results in discomfort through either being squashed into a window or having to sit on half a seat whilst those in the aisle make every attempt to remove my arm. Sometimes it's better to stand!
In case you're wondering, yes, I do always ask if someone minds me taking a seat next to or opposite them - it's basic manners and I won't be offended if they say no without giving a reason, they may be waiting for someone they are travelling with or they may just not like me - I don't really care, I'll get to my destination and that's all I can really expect. I don't particularly want to sit next to somebody who doesn't like me, it just makes the situation worse for everyone. Most of the time, even on busy trains, there's a seat somewhere else.