No. They dont have to make the effort to change seats on a moving train just to accommodate an immediate and false sense of personal space made available by the empty seat next to them.

I wouldn't personally tell them to move over, but that's only because I'm the sort of person who greatly dislikes confrontation. If there was another seat available, and it was the sort of train where switching seats is OK, I'd move myself; otherwise, I'd scoot away as far as I could. Last resort, I'd get an attendant to sort things out.

If you were sitting there first I would kindly ask them to please scoot over a bit ...or maybe ask the conductor to ask them.. I don't like people leaning on me at all. Especially on public transportation

I actually had this problem once.. My friend fell asleep and took over my seat as well as hers.. I did politely try to ask her to move over.. But she was sleeping.. So what I did is I spent the rest of the night in the cafe car playing cards, and obersation car.. The conductor wondered why I was not sleeping.. Well.. I can not sleep on Trains I love trains so I want to be awake for every moment to experience it.. Someone else told him my friend was hogging the seat.. He offered if that happens... Just go and ask the conductor.. and he'll politely ask them to move over, or change the seating arrangement.... I was on Amtrak when that happened. But since my peers kept telling him how I wanted to work on the railroad. I had fun helping them pick up a few minutes by helping carrying on and off luggage on the route.. Not to mention.. The extra things I got to see from the observation car, and the new card games I learned in the cafe car.. It was a fun trip.. But if you are on a train where a passenger is hogging the seats if you are to embarassed to politely ask them yourselves for a little room and space.. feel free to ask the conductor after explaining the situation (politely) and they will help you out. :)