I just threw a thomas party for my little boy. So I did a lot of digging to find good ideas. A lot of them didn't get used because of how I forget that this small town doesn't carry much and I shopped last minute. But I thought I would share the ideas here.
One was to get inexpensive painters hats and draw stripes on them to make conductors hats. I was planning on having the kids decorate them as there parents dropped them off.
Play don't eat THOMAS! Played this with a card that had a bunch of Thomas photo's on it like a bingo card. Then we would place an M&M on each square. The objective is for the child to eat all of the M&M's before they find Thomas. When they go to eat the M&M which you have selected to be Thomas everyone yells DON"T EAT THOMAS, and then it's the next child's turn.
Use boxes to make into train cars, even have the kids decorate there own train car. Give them popcorn and throw in a Thomas DVD and they can watch Thomas while in there cool train box cars.
Do a bike parade and have the kids decorate there bikes. Then have them line up like a train and show off all they have done.