Remove thawed cakes from containers and trim each to a 7- x 3-inch rectangle. To make the engine:
In a small bowl, mix 1 1/2 cups of frosting with about 4 drops of blue food coloring until well blended. Place one pound cake on a clean work surface and cover with frosting. Divide another pound cake into two-thirds and one-third pieces. Place the smaller piece on top of the frosted cake at one end to make the cab. (Reserve the larger piece for the caboose.) Frost the cab evenly, filling in any cracks where the two cakes meet. With a wide offset metal spatula, carefully transfer the engine to one end of the serving platter or board.
To make the log car:
In a small bowl, mix about 1 cup of frosting with about 2 drops of yellow food coloring until well blended. Cover one pound cake with frosting. Place behind engine on platter.
To make the caboose:
In a small bowl, mix 1 1/2 cups of frosting with 4 drops of red food coloring until well blended. Cover the last whole pound cake with frosting. Center the remaining piece of pound cake on top and frost evenly. Top with the candle and place caboose behind log car on serving platter. Serve immediately, or cover loosely with plastic wrap and refrigerate up to eight hours.