For the train engine, cut one cake into a 6-inch piece, a 2-1/4-inch piece, and a 1-1/2-inch piece. Cut the 1-1/2-inch piece in half horizontally. For the two train cars, cut other pound cake in half crosswise. Tint 3/4 cup frosting red, 3/4 cup frosting yellow, and 1/2 cup frosting green. Cut thick pieces of cardboard to fit under engine and cars. For engine: Frost top and sides of 6-inch piece of cake red. Frost 2-1/4-inch piece of cake red on 2 sides and yellow on 3 sides. Place on large cake piece to make cab. Frost 1-1/2-inch piece red, and place lengthwise on engine. To make cars, frost top and sides of 2 remaining cake pieces -- 1 green and 1 yellow. Place frosted cake pieces on cardboard.