For On30, I tend to use the Bachmann coupler as long as it has the metal coil spring on the knuckle, if the car came with a plastic leaf knuckle spring, it gets tossed in favor of a Kadee. Since the advent of Kadee's whisker spring, that's what I prefer to use when switching to Kadee's, but I still have a lot of equipment with the old flat phosphor-bronze centering springs. The Bachmann coupler boxes seem to be a few thousandths of an inch shallower than the Kadee's, enough so that the screw holding the lid can't be fully tightened, or else the Kadee won't return to center. If the screw keeps working loose, allowing the coupler to fall off, filing the Kadee shank so that it's thinner may work, or use the Kadee box after removing the molded in box on the car frame.
Bill in FtL