I made this train birthday cake for my son's 3rd birthday. I used a big loaf pan and 2 smaller ones which worked out to be the same volume as 2 regular cake pans.
I made 3 batches of cake, ending up with 3 big loaves and 6 small ones. For the engine, I used a big loaf on the bottom, topped with 1/3 of a big loaf for the cab and 2/3 of a small loaf trimmed to make a cylinder for the boiler.
The coal car is 1/3 of a big loaf hollowed out in the middle. The passenger car (blue) is 2 small loaves stacked on top of each other. The flatbed car is a small loaf hollowed out in the middle. The caboose is 1/2 a small loaf with another piece on top trimmed to have a roof with a slight overhang.
The brown car has chocolate buttercream frosting, but for the rest I used gel paste food coloring, which I just discovered and love. I also learned how to smooth buttercream frosting: a couple minutes after spreading the frosting will crust, then you use a paper towel and smooth it with the warmth of you hand, then carefully peel off the paper towel. (Maybe everyone already knows this!)
The engine wheels are licorice in the front and full Oreos in the back. The other cars have single Oreo wheels and are linked together with lengths of stick licorice. The coal is crushed Oreos with black sprinkling sugar, and the cargo in the flatbed car is fruit shaped candy. The other trim is writing icing. (It says "Auguri" rather than "Happy Birthday" because we live in Italy.)
I added a picture of my son in his engineer's hat as the driver, and pictures of his friends as passengers in the blue car, stuck on at the last minute with a little frosting.
My son loved it and all the kids were excited to pick which car they wanted their slice of cake to come from.