Invitations: A train ticket/boarding pass would make a cute invitation.
All Aboard! It's [child's name]'s Birthday!
Chug on over to the [Last Name] Station
Boarding Date: (date)
Departure: (start time)
Arrival: (end time)
Regrets Only, (ph #)
Activities/Games: It would be fun to make everything a "station".
Coloring Station - Check out ThomasAndFriends.com or simply search for "thomas the tank coloring pages" on Google Images. Let the kids color as they arrive.
Loading Station - Ahead of time get some Thomas the Tank fruit snacks. Then using brown craft paper, wrap them into a package with several layers. For the game, have everyone sit in a circle. Play some music (from Thomas soundtrack, if available), and have the kids pass the package. When the music stops, the child holding the package gets to unwrap one layer. Continue playing until finally the last layer is unwrapped and everyone gets a "prize"... aka, the fruit snacks.
Sir Topham Hatt Station: Like Simon Says, except the instructions are "Sir Topham Hatt says Go Fast", "Sir Topham Hatt Says Go Slow", "Sir Topham Hatt Says Sound Your whistle", "Sir Topham Hatt Says Go to Sleep", "Sir Topham Hatt Says Go Chug-a-Chug". And for little ones, if they don't grasp the concept of "Simon Says", then give the instructions as the kids march on the "track" (masking tape). Then, when they reach the end of the track they get a Thomas Sticker or something.
Art Station: If you're really brave, you could do this. Get wooden craft trains at the craft store. On some Styrofoam plates, squeeze a few little dollops of washable finger paint. Give each child some poster paper, and let them take their wooden trains, run it a little through the paint and then make tracks on the paper. Be sure to line your table with newspaper. Best of course, if this activity is outside. Also, have some baby wipes available for fingers.
Troublesome Truck Station: At one end of the room set out some empty shoe boxes, decorated to look like train cars. Then at the other end have a bunch of balled up black socks on the "track". Explain that some Troublesome Trucks have dumped their "coal" all over the tracks. Let each guest take a turn to see how many balled up socks they can toss and make into the basket. Give them a point for each one that makes it in. Then, hand out something fun for everyone who "helped clear the tracks", like a red conductor's bandanna or something.
Follow-the-Whistle: As a grand finale game, hide the "loot" somewhere in the house. Have an adult blow a train whistle. Tell the kids, "Do you hear that? I hear a train whistle! Let's see if we can follow the whistle." Then have all of the kids go throughout the house until they find the whistle, accompanied by their final prize. A final prize could be mini-train whistles. You can find these at Oriental Trading (click here) - $10 per dozen. I am sure they have tons of other train crafts, etc.
Food: Choo-Choo Chili or Sub Sandwiches, which are like "train cars" - use cucumber slices for wheels. You could also keep it simple with Chugga-Chugga Hot Dogs , Choo-Choo Chips, and Toot-Tootin' Root Beer.