just got my son a Rail Chief set for Christmas and the back set of wheels r locked. the front wheel set is spinning on the track and is even moving the train very slow, but when i look ground level i can see the front spinning and back is just locked dragging. Is the engine broke or is there something i could do to loosen the wheels? i do have the train under power (i know they don't turn by hand). Any info would be great bcuz my son can't wait until he can couple the other cars to the engine.