Hi!
When I was at uni I had a hammy called Maizie and she once came on the train with me from Bangor to Rhyl, and once from Bangor to Stockport.
Arriva trains wales seem to allow hammy passengers, and changing at chester, i think it is first north western and they were ok with it too. The conductor was really interested in her!
Maizie was really nosy and interested in everything that was happening, but eventually went back to sleep.
I think it really depends on the temprament of the hammy in question, but I do agree, keep him warm, and leave him to his own devices
Definately reserve a seat, and as long as it is only a one off, he should be ok
My boys are having the house to themselves xmas eve as we are travelling to my parents back in Wales for the night