What scale? I'm modeling ho in a modular club. We frequently will run some long trains, and have found that the KD's work best.
The various KD knock offs all seem to use a plastic leaf spring to hold the knucle closed, and when you put a load on it, the knucles will open and uncouple. I bought a Bachmann Spectrum consolidation at a GAT show, and wanted to run it right away, so I used the standard Bachmann couplers. We were doing some switching with it and never pulling more than 6-8 freight cars and a caboose, and in a couple of hours the couplers started to "stretch." When I got home from the show, it got KD's.