I was just running some new (to me) Rail King heavyweights and having the usual trouble with a coupler opening spontaneously. With modern plastic thumbtack armatures, I usually get improvement by bending the plastic a little to put more force on the pin. This also seemed to help with metal ones, I assume because it allows the armature to get closer to the coupler body–the spring is separate from the armature.
This time I played with the coupler and finally succeeded in getting it to open simply by wiggling the knuckle up and down while trying to pull it open. It just walked the pin right out of the coupler. I took a little file and made a shallow notch at the base of the pin, just the width of the ledge on the coupler body that it bears against and on that side of the pin, only about 1/16-inch wide. Now neither I nor my locomotive can work it open; but the uncoupler track opens it just fine.