I recently purchased a Lionel 746. It appeared to be in good condition but when I ran it sometimes it would stop running and sit there and growl in either forward or reverse.
I pulled it apart thinking something was binding but found nothing. I hooked it up to my test transformer with the motor out of the engine and the motor would run most of the time. Finally it stopped and started to growl just like it did when inside the engine. It turned out that a wire from the field coil was shorting out to the motor case. It seems like when the motor was made, many years ago, the field winding which went to the input lead wire was left a little long and it was rubbing on the motor chassis. It had worn thru the insulation and when there was some vibration it would short out.
If you have a motor that is getting power but is just sitting there growling check your field coil.
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