Uncouple Issue

Depending on the type of couplers you have:  Sometime an uneven track will cause one coupler to move downwards in the other, and trigger the armature.  Or the pin could not be going into the housing far enough to lock the knuckle.

Thank you for your reply. I`ll be looking at that.

Two thoughts come to mind:

1. Check the coupler height on the tender, including trying to move it vertically.  If uncoupling is occurring without any open knuckles, then it is definitely vertical bypassing of the couplers.
2. Do you have any grades on your railroad?  Perhaps the uncoupling occurs in areas where the grade is in transition?

Even with oversized couplers, it is possible to have height mismatch, so even a small track imperfection can cause this. I recently had to do a coupler adjustment on a passenger train for just this reason. 

Does the uncoupling happen in the same location on the layout?

Thank you all for your replies. I will look into all your suggestions. 

The above comments are spot on. I had the same issue with different ars. On one, the coupling was loose and would rise up and the two couplings would be uneven, and uncouple. On another the coupling of one was actually larger than the other so the "mouth" or space made it loose and it would sometimes uncouple going around a corner.   Two fixes I did, were add a piece of black tape to the coupling finger on the larger one...like you might on a ring on your finger, and that tightened it up better. The rising coupling, well, I added a thin piece of black plastic to the bottom of the tender so the coupling wouldn't rise so much. You's could be different but that was what I found. Good luck.

Thank you, all your comments are appreciated.