My boys have loved Thomas the Tank for almost 10 years. In that time I have made 3 Thomas cakes and 1 Percy. They always turn out so cute and the kids LOVE them so I thought I'd give you a quick "how to" in case your little engineer would enjoy it as much as mine have.
Start out with a 9x13 cake cooked according to the directions in a well greased pan. Freeze it after giving it just 15-20 minutes to cool. I put the whole cake in the freezer pan and all. Freezing it not only makes decorating it so much easier but it traps the moisture and makes the cake really moist.
Once it's frozen through take a sharp knife and cut it to this shape. I typically shave the top edge off one side and the turn it and use it as a pattern to cut the second cut. It makes it more uniform. Keep the pieces. Use a small bowl or large coffee mug for a pattern and cut a circle out of the center of the top section. Gently pry the circle out and place your cake pieces in the hole. Then replace the circle. It should stick up about 1/4" from the surface of the rest of the cake. Put the cake back in the freezer between steps.
Most of the pics have a smudge on them. Apparently I got a little of the icing on my camera. I apologize :)
Use a base frosting (3 cups powdered sugar, 1/2 c shortening, 1/4 c milk, 2 tsp clear vanilla extract) and thinly coat the cake.
You will need professional food coloring to the colors you want for this cake. At Wal-Mart they are in the craft department not with the baking. You need Black, Red, and Blue. Use a cake decorator's frosting (1 1/2 c shortening, 1 1/2 tsp vanilla extract, 7 1/2 c sifted powdered sugar, 1/2 c milk). I do a lot of mixing to get the perfect color. I would make an entire batch the black. While trying to get a rich black stop once you have a nice gray and pull out about a cups worth and sit aside. Then continue until you have the color you want. Outline the cake like this using a star tip. Remember to continue the lines down the sides of the cake.
Now make another batch of the decorator's frosting (see above) and mix the blue. Go easy on the coloring. Thomas is a teal blue not a royal and professional coloring is super concentrated. Once you have the color you want fill in the top of the cake like this. Continue with the star tip.
Now switch your tip to a Bismark tip (small hole) and draw the lines of his face with the remainder of your black icing.
I usually make a 1/2 batch of the decorators icing for the red. Switch back to your star tip. You don't need a full batch. The red coloring will not make a true red. Mine always looks a little orangie pink in the bowl but in contrast on the cake it looks perfect. So don't get frustrated if it doesn't look red enough.
Finally fill in the face and the last line of his bumper with the gray. And viola!