A little history here. Back in the 1950's the National Model Railroad Assn (NMRA) designed the horn hook coupler. Athough they failed to get it approved as a official NMRA design, the industry adopted it because it was cheap to manufacture (one molded plastic part) standard (would couple with other makers rolling stock), looked OK, and no patent restrictions or royalties. About the same time Kadee introduced their knuckle coupler. It was patented and Kadee wanted $2 a pair for them so the industry never adopted it as too expensive. But everyone agreed that Kadee's looked better, and worked better, especially remote uncoupling. In the late 1990's Kadee's patents finally expired, and the industry started making clones of Kadee knuckle couplers, which would intermate with Kadees and uncouple on Kadee magnetic uncoupling ramps. At this time all new production rolling stock comes from the factory with clones of the Kadee coupler. They are pretty good, I run them until they break.
Last time I looked, hobby shops all carried Kadee couplers, either two pair packs or 20 pair packs at about $2 a pair. They sometimes carry clones of the Kadee coupler for the same price. So why not buy the best, original Kadee's?
As others have mentioned, a #5 Kadee or the whisker equivalent will fit most freight cars. It is possible to warp a #5 coupler into anything, but you will save a lot of filing, hacking, and cussing if you check the Kadee what-fits-what list and buy the recommended coupler, especially for locomotives, passenger cars, and long rolling stock. Be aware that locomotives need electrically insulated couplers, either all plastic couplers, or an all plastic coupler mounting box.
Most people replace truck mounted couplers with body mounted couplers. Body mount couplers prevent derailments while backing. I retain truck mounted couplers on passenger cars and other long rolling stock because truck mounts ease the long rolling stock around sharp (18") curves where body mounts tend to pull the car off the track.
Good luck.
