I have a Lionel an 8690 Trolley, it was running great up to a couple of weeks ago.  The trolley started to stop suddenly in one direction. It appears there is a bind in the wheels.   I cleaned and serviced the motor, checked for bent axels, put new brushes in and lubricated it as per manual.  It still stops in one direction but runs great in the other.  The wheels still lock up as though there is bind in the motor, axels or gears, but I cannot find anything out of alignment.  Does anybody have any suggestion?