Cream shortening; gradually add sugar, beating well. Add eggs, one at a time, beating well after each addition.
Combine flour, cocoa, baking powder, soda, and salt; add to creamed mixture alternately with buttermilk, beginning and ending with flour mixture. Stir in vanilla.
Pour batter into a greased and floured 13- x 9- x 2-inch baking pan. Bake at 350° for 35 minutes or until a wooden pick inserted in center comes out clean. Cool in pan 10 minutes; remove cake from pan, and cool completely on a wire rack. Freeze cake about 2 hours. (Cake will have clean edges when cut for assembly into train engine.)
Cut cake into two 10- x 4 1/4- inch rectangles; cut remaining cake into two 4 1/4- x 2 1/2-inch rectangles. Spread Chocolate Frosting over top of one 10- x 4 1/4-inch rectangle. Top with remaining 10- x 4 1/4-inch rectangle; frost top, and set aside.
Align edges of two remaining 4 1/4- x 2 1/2-inch rectangles to form a 5- x 4 1/4-inch rectangle. Spread Chocolate Frosting on cut edges of center seam to attach. Using a large metal spatula, carefully lift rectangle and place on top and at one end of larger frosted rectangle, aligning the 4 1/2-inch edges. (One end of cake should have 3 layers arranged to resemble an engineer's station at rear of train engine.) Trim sides of cake to make layers even, and remove any excess cake crumbs, if necessary. Spread remaining Chocolate Frosting over top and on all sides of cake.
Place about 2 cups Decorator Icing in a small mixing bowl. Add yellow food coloring to reach desired tint, mixing well. Reserve remaining icing for additional colors, if desired.
Fill a pastry bag with yellow Decorator Icing; pipe icing attractively onto edges of frosted cake and outline details (such as windows) to resemble a train "engine."
Using Decorator Icing, attach 6 peppermint rounds to 6 vanilla wafers for wheels. Connect wheels with licorice whips; secure to engine with Decorator Icing. Cut away lower portion of the sugar cone; discard larger end. Decorate cone with icing, and insert at front of engine to resemble smoke stack. Place a peppermint round in center of front of engine. Pipe icing attractively around peppermints and engine wheels. Use candy dots to decorate cake as desired. Serve cake with ice cream.