Decorate cake. Using knife, trim bottom of cake to sit straight. Using teal icing and angled spatula, ice train top and engine smooth.
Using yellow icing with tip 5, pipe windows and top of smokestack. Smooth with spatula. Using same bag, pipe outline around windows.
Using bright green icing with tip 8, pipe band around top of train; smooth as needed.
Using bright green icing with tip 5 and pastel green icing with tip 5, pipe lines to cover smokestack. Using spatula, smooth colors together.
Using pastel green icing with tip 5, pipe horizontal lines on bottom half of body of train and on back engine.
Using light orange icing with tip 5, pipe vertical lines on front of train (cowcatcher).
Using medium blue icing with tip 12, pipe wheels. Smooth using tapered spatula, leaving center of wheels open. Pipe center using light orange icing with tip 5.
Insert meringue cookies into top of smokestack.