We've been 3 times now and have enjoyed it each time. It has definitely gotten busier each time, so you may have a little wait for some things like Bulgy the bus (Bertie is the smaller one

).
Some of the other characters they have had:
Thomas (of course!): a full sized steam train with trucks and carriages that you can ride in. Like Rackhell said, you just go back and forth but the kids think it's great.
Trevor: An old traction engine you can look at and take photos of.
Terence: A tractor on caterpillars. Again to look at and take photos with.
Rusty: A little train, kids can climb in and play with switches.
Bulgy: A double decker bus you can pay to take a ride in. The driver is a real character and it's great just listening to him talk, plus you get to see some of the beautiful countryside.
Caroline: Haven't actually seen her!
I do think the 30min train ride is worth it. You get to stop half way and check out a (maintenance?) shed full of other trains (well my DH & DS thought it was great!). The kids have a blast and talk about it for months.
We take food and have a picnic. From memory there isn't a great range of food for sale. There is usually other entertainment like Rackhell said, bouncy castle, face painting, a bubble blower and there is a playground for the kids. Also there is a massive tent with Thomas merchandise for sale.
Next door is actually a little farm you can visit - haven't actually been but perhaps something else to think about?
I'd suggest you get there as early as you can to avoid the worst of the crowds. As for the grandparents, I'd let them know that there may be crowds but it's about all being together to enjoy your DS's birthday (I'm trying to avoid saying 'It's not about you so get over it!'

)