I have six of the flyer Chief Berkshires and five have had this problem while running usually causing board failure. They all have been to Lionel for repair and one just came back with a retainer missing.
They should have redesigned this flimsy part on the new production but from the above posts that did not happen. I will be very happy when AMERICAN MODELS gets my new pacific's in.
They test run em before shipping and the owner answers the phone!
Gunny