No it's not too babyish. I used to have a cake book that showed you how to make different cakes for different occasions. I remember one with a train - you bake a regular cake mix (or 2, depending on your guest list) in those small mini-loaf pans. (You could make a large cake and then cut it up, but it makes frosting it very difficult on the cut edges.) Unmold the little cakes and line them up, train style, on a large platter or even a board you cover in foil or a section of a plastic tablecloth. Frost each section in a different color. Add 4 little oreos or any single-layer chocolate cookie to each car to make wheels. Connect them with a section of black licorice. Take one small cake and cut it in thirds - use one piece to make the locomotive taller in the back (attach it to the lower cake with a smear of frosting, then frost the whole thing. Take another small section and make it the "coal car" behind the locomotive. Take the 3rd section and make a little "roof" on the caboose. Use candies, small crackers or small square/rectangular cookies to make windows in some of the cars. You can also use gel icing to make letters on the sides of "freight cars". It's kind of fun and then each kid can choose the section of cake they want - usually half a "car" is fine for each kid.
I'm sure you can find this sort of thing on line with a photo.
Give the kids little engineer hats from the party store. You can buy a single train whistle at many toy stores or anyplace that sells to teachers - use that to start and stop games. You can adapt almost any party game to the train theme - pin the stack on the locomotive, etc. You could have them decorate wooden or foam picture frames with train-themed stickers and shapes, then take a photo of the group of "engineers" and email it afterward to the parents. You could have a treasure hunt, hiding little boxes around the house or yard, and have the kids "locate the missing train cargo" - whatever they find becomes their prizes/goodie bags so you don't have to do separate goodie bags. Just get the bags for collecting their loot. If you can't find train-themed bags, get clear ones and put train stickers on them. Craft and scrapbooking stores have tons of that stuff. Kids don't really care what you do as long as you make it fun!
