Line up cakes on large tray or serving platter for train cars.
Cut 2-inch-thick crosswise slice off front cake; place at back end of the first cake to resemble train engine. Repeat with last cake, placing the removed cake slice in the middle of the last cake to resemble caboose.
Divide whipped topping among four small bowls; tint with food colorings as desired. Use to frost cakes. Add decorations as desired. Store leftover cake in refrigerator. Makes 24 servings.