Gents......
Over the years, I have purchased products made/imported by ACGilbert, American Models, Lionel (O), Flyonel (S), Marx, SHS, DPH, "S"cenery Unlimited, NASG, ALCO Models, SouthWind Models, Sunset Models, Overland Models, River Raisin Models, Funero&Camerlengo, Smokey Mountain Model Works, Kinsman, Mullet River(?), Delaware Valley Models, Lehigh Valley Models, BTS, PBL, Mantua Metal Products and about a dozen other companies who have made various kits from time to time. And that is just for stuff that rolls on the track. Counting structures and scenery items, the number of manufacturers represented on my layout is probably ten times greater.
Early on, the need for standardization was obvious. Couplers, track, wheels, etc. varied all over the map. Nothing was assured of working with anything else. And that, my friends, is the bane of S and is what scares off converts from the other lesser scales. So I turned to the NMRA and standardized on their wheel and track specifications. As a result, I use code 100 rail, NWSL wheels and Kadee #5 couplers on just about everything possible. The main exceptions are brass imports and locomotives where changing the wheels is very difficult. But I have certainly re-gauged loco wheels over the years to become compliant with the NMRA standards. Today, everything on my layout plays happily with everything else. But happiness did not come easy.
One thing for sure is that "S" does NOT stand for standardization.
Cheers.......Ed L.