Some other ideas:
- Red bandanas make great party favors to go with a little train conductor hat or (if you are brave enough) whistle!
- Make train boarding passes for the invitations instead of buying them, then you can have hand stamps for when they enter
- Make cardboard signs to use as decorations (such as stop signs, RR crossing, etc)
- Set up little play areas where the kids can rotate through playing with different train sets
- Make a duck tape railroad track for the kids to play on. (just be careful what you stick it to, I'd stay with carpet!)
- MUSIC!!! Play music and/or videos in the background to help keep the mood going.