1. Frost cupcakes. Cut red licorice into 6 (2-inch) pieces and lay 2 across top of each cupcake, for tracks. Cut red licorice into 6 (1-inch) pieces and lay 2 inside each set of tracks, perpendicular to first set.
2. Squeeze icing on bottom of 1 (8-stud) brick and stack on another, for train. Repeat with remaining 4 (8-stud) bricks to make a total of 3 train cars. Squeeze icing on bottom of 1 (4-stud) brick and stack on first car, for engine car. Squeeze icing on bottoms of 4 (2-stud) bricks and stack, side by side, on second car. Repeat with remaining 2 (4-stud) bricks, stacking on last car.
3. Squeeze icing onto one side of each regular M&M and attach 2 to each side of each train car, creating 4 wheels per car. Place 1 train on each set of tracks. Cut black licorice into 1 (1-inch) piece. Squeeze icing on one side of Mini M&M and attach to black licorice, for stop light. Push stop light into engine-car cupcake in front of car. Makes 1 train with 3 cars.