John,
A thumbtack coupler opening by itself? Preposterous! 
When Lionel went to those 2railer conversion plastic trucks and the thumbtack coupler the "Kool-Aid" drinkers raved about how much better they were than the metal trucks they replaced.
Had to tinker with a few and we have had some out of the box issues with the couplers dropping on the 86' cars because of the tiny piece which rides above the floor dropping down. They are a bear to fix when that happens and the car we sent back was returned with damage and not repaired so rather than play roulette we have reduced the fleet down to 2 and stopped adding. Like Paul's running situation, they now reside at the end of trains.
