I purchased Lionel Polar Express set # 6-84328 last March and it ran fine when I tested it and has been running fine through the holidays but yesterday the chuffing sound began to operate only intermittently in the forward direction while it still sounded perfect when running in reverse. I am running on Fastrack and have thoroughly cleaned the track as well as the locomotive wheels and its pick up rollers. I also made sure the little metal tab attached to the rear truck was making good contact with the truck's axle and cleaned the axle as well. Still not much chuffing in the forward direction but fine in reverse. I'm not sure if there is anything else I can do to correct this problem but if anyone has any advice or recommendations I'd appreciate it. Thanks in advance for any responses.