I've seen hump yards with models, that worked almost flawlessly, and others that wouldn't, no matter what you paid them. Ed Ravenscoff (SP) had a hump yard that was extraordinary! Not only did it work extremely well, it was curved, using air jets to slow the cars down, (I've also seen paint brush bristles used as retarders. These were removed from the brush and glued parallel to the track on the inside, and rubbed against the wheels and axles. The stiffness of the bristles and the length that was glued down determined how much the car was slowed.), but he also used horn-hook couplers exclusively. He even placed a wire that ran from the the "inside" of the coupler, via an eye bolt to the side of the car. Every car had this set up, and to uncouple, all Ed had to do was squeeze the wires toward one another and the couplers would uncouple. The end of the wire was bent to look like the end of the brake lever, that you see of every prototype car out there. As a system, Ed had a real good one.
The secret to this he told me once, (at a convention, I never actually saw his layout.) was the make of the horn hook. It had to be metal, and it had a small bronze spring the pushed against one side of the coupler, so the gathering face of the horn hook was always pushed slightly to one side. The couplers also had to be body mounts, or if truck mounted, the trucks would become offset when the train was pushed, and a derailment was almost guaranteed. The brand name that he used was Rail-Line's. They had several versions of their couplers on the market, and could be bought 2 pair to the envelope, just like KD's.
He didn't use KD's at the time as there was only one KD on the market, and those weren't that reliable. Instead of that bend in the knuckle that keeps the modern versions coupled, the knuckle was "straight faced", and side to side pressure could cause the cars to uncouple. While they did have a slight inward slant on the inside face of the coupler, it didn't help much to keep them coupled. They also didn't uncouple with magnets, there was a straight pin imbedded in the knuckle and it used a ramp to move the knuckle to one side for uncoupling. Their next coupler was known as the MKD, greatly improved, and it eventually became the KD #4.
Sorry for the Ramble, but I feel that the couplers on the engines and cars also have a lot to do with the success or failure of a hump yard, and not just how well the cars roll.
