I'd shift if it was a long journey, probably.

Stay put if it was owt less than about an hour.

If its a fat person or a smelly person next to me then I move like a shot!

If its not and I am by the window then I stay put....but if not by a window and a window seat becomes available then i move.

I'd move seats.  I like my own space.

I don't really travel much by train, and when or if I have, it's never been overly packed.
That said, I'd move.  I'd also mutter some comment about giving each other more space and smile.  Like this >>>> .

I have a charming smile and therefore offend no one. 
The other person would thank me profusely whilst regretting not having me next to them.

I'd move, I can't abide my space being invaded which is another reason why I don't travel too much on public transport (apart from picking up infections, not having the puff to get from one platform to another etc.

Yes you see that is what I would do cos I do like to spread out 

It just fascinates me when I see a half empty train and two people sitting next to each other obviously uncomfortable. I generally move but just wondered if it appeared rude?

I think it's an unspoken courtesy that people expect to move, Issy.

Last time I was on a train (the Gatwick Express) I didn't move Wild horses wouldn't have got me shifted. I was diagonally opposite and across the aisle from Jo Wood.  I gawped all the way

Cosi....

Who is Jo Wood?

Wot SB said 

She was on Strictly, Issy.  The ex Mrs. Ronnie Wood.
I was listening out for any goss...........

OHhh

ah yes I remember now.

It annoys me, when I'm on bus or train and there's millions of empty seats around me, someone gets on and plonks theirself next to me   I feel like saying will you P Off  go away 

This is what me and my buds do anytime we get up to London.

That ALWAYS happens to me as well Sprout while the jammy so and so opposite me takes up 2 seats...

I just love the deadpan 'we are not amused' looks on the passengers faces 

LOLOLOL Rawky - that really made me laugh.
Love the way the passengers did the typical British thing... pretended it wasn't happening and then politely applauded at the end..

That was brilliant Rawky 

 
They'd probably get a better reception at the notorious Glasgow Empire

Oh, I've met some v interesting people on trains who I'd never come across otherwise, (bit like here) so I'd stay if they were interesting , and go if they were doing that "Don't speak to me" vibe

If the train was empty and I was sitting by myself and someone came on at the next stop and sat next to me I would wonder why, but I would not move to another seat unless they were making a nuisance of themselves,then I would start to worry and probably try to move to another part of the train

I would possibly move, whilst muttering something about having to spread out, and thrush.