How do you interchange knuckle couplers and rapidos? Just recently got back into model rr and have tons of stuff from the mid 90's all brand new in the box.
you can swap them for replacement trucks and couplers(http://i1256.photobucket.com/albums/ii494/chesapeakeandohio/mtl-00310031-2.jpg)these from micro trains.
You can make a transition car, or two, or more. To make a transition car you replace the truck and coupler on one end and leave the Rapido coupler on the other. I would suggest changing all your cars over to knuckle couplers, but with transition cars you can run them all and take your time with the conversions. MT trucks and couplers are expensive, but worth it in the long run.