Here are some links to train cake pans, the one at Williams Sonoma is no longer available but the decorations on the cake in the picture are so adorable that I put in the link. I get inspired by seeing pictures like this when I am trying to get creative.... Anyway, the pan is available on Ebay or other sites. The Wilton cake pan is also very cute and you can find it on Ebay or the Wilton website.
You could use different flavors/colors of cake in each compartment of the little trains for more variety. If many are coming the party you might want to make more cars, each person may get 1 car, I am not sure of their exact size.
It sounds like a very fun party!!!!!! Let us know what you decide, my J loves trains too and this is a good idea.
Good luck!
http://www.trainparty.com/traincakepans.html
http://search.yahoo.com/search?fr=ush_mailm&ei=UTF-8&...
http://www.williams-sonoma.com/products/sku8239147/index....
http://shop.ebay.com/items/_W0QQ_nkwZcakeQ20pansQ20trainQ...
http://www.wilton.com/shapedpan/Choo-Choo-Train-Pan