I recently acquired 4 MTH RK UP semi scale boxcars (30-7412) to go with a recently acquired UP locomotive. A coupler on one of the cars opens on its own before the train even gets half way around the loop. I have Fastrack and there are no uncoupling or operating sections in the loop. I can't see any problem when I look at it on the bench. The coupler mechanisms seem tight. I might detect one end having a looser spring tension but I can't be sure.
Any suggestions on what to look for and how to solve the unwanted uncoupling would be greatly appreciated.
Thank you
David