Make cookies one day in advance. Prepare and roll out dough following recipe directions. Use Thomas cutter from set to cut shape. Bake and cool cookies.
Outline and flow in cookies 1 day in advance.Prepare Color Flow icing following recipe directions. Tint portions blue, yellow, black, gray (use black icing color) and red. Reserve a portion white.
Use tip 3 and full-strength Color Flow icing to outline gray face; black engine, smokestack and undercarriage; blue cab; and yellow windows. Let dry 1 hour.
Use thinned icing in cut disposable bags to flow in outlined areas. Let dry 24 hours. Reserve remaining icing.
Decorate cookies. Use full-strength icing to decorate all details. Use tip 3 and red icing to outline and pipe in red running board. Pat smooth with finger dipped in cornstarch. Use tip 2 and white icing to pipe in mouth and eye. Use tip 2 and gray icing to pipe outline facial details. Use tip 2 and black icing to pipe outline eyebrow and vents, dot pupil and coal.
Use tip 3 and blue icing to pipe outline spokes and dot hubcaps. Flatten and smooth hubcaps with finger dipped in cornstarch. Use tip 3 and gray icing to pipe outline wheels and wheel rods. Use tip 3 and gray icing to outline and pipe in bolts on wheels. Pat smooth. Use tip 2 and blue icing to pipe dot screws. Use tip 3 and yellow icing to pipe number 1. Let dry 1 hour.