Using serrated knife, cut rounded top off each cake to level surface; place each cake cut side down. Cut first cake as shown in diagram to make engine, being careful not to cut all the way through piece 1 when removing piece 2. Place piece 2 on top of piece 1 as shown in diagram, with a small amount of frosting between pieces. Cut second cake into 4 equal pieces to make cars as shown in diagram. Spread thin layer of chocolate frosting over top and sides of engine to seal in crumbs. For each of the car cakes, spread thin layer of red, yellow, blue and green frosting over top and sides of each car to seal in crumbs. Refrigerate or freeze cakes 30 to 60 minutes or until frosting is set.