Here are the instructions for the Thomas the Train cake I posted about recently.
1 - 11x15 inch sheet cake
1 - 6 inch round
Fondant in blue, black, red, light gray, dark gray, white, yellow
Level the 11x15 inch sheet cake (keeping the roundy top) and cut where all the black lines are on the image below:
The gray area becomes your main cake base. Frost the base and cover in blue fondant.
Piece No. 1: I took a piece of the rounded cut off top and cut it the same size as the piece to make it a bit taller and to give it a rounded shape. This is optional. Whichever you choose, frost the piece and cover in red fondant.
Piece No. 2 (2): Cut both No. 2's with a round on one side to match the curvature of the 6 inch round. Frost both pieces and cover each one in blue fondant. I left the side that would be touching the round without fondant.
Piece No. 3 (2): Frost and cover each with blue fondant.
Piece No. 4: Cut the ends into rounds (see image below) and cut the piece on an angle so it is taller on the side that will butt up against Piece 1 and shorter on the side towards the round. I actually cheated on this one too and ended up just using another piece of the roundy cut off top. It had the perfect slope already. Frost and cover in light gray fondant. I left the side that was aginst Piece 1 without fondant.
Piece No. 5 (2): Glue both No. 5's together vertically with frosting. Cut the bottom end to match the curvature of the 6 inch round. Frost and cover with black fondant. I left the rounded end fondant free.
Piece No. 6 (2): Frost and cover each with dark gray fondant.
6 Inch Round: Level off. Frost and cover with black fondant. Place circle of light gray fondant on top. If you are really skilled, you could cover the round in light gray fondant and wrap a band of black fondant around the outside. I'm just not that good.
Assembling the cake: Place the pieces as indicated on the diagram below.
Add accents as in picture below.
The 4 on the smoke stack is the age of the birthday boy and not necessary. I actually placed the piece of black fondant between the round and Piece 4 before I placed those pieces on the cake. I used random rounds lids I had to cut out the various circles. I freehanded the mouth and eyebrows and used small, rolled pieces of black fondant to make the face. You could pipe it as well, but again, I'm just not that good.
I'm pretty sure you could also use a 9x13 sheet cake and a smaller round (just cut down a bigger round if needed). You would just have to cut everything smaller. Frosting and fondanting all the little pieces was a bit of a pain. If you have the time to freeze the pieces first, it would make it much easier.
And that's it! Piece of cake!