I can immagine that since the RGS was such a poor railroad at the time, the lease of a K-28 would have blown up their bugdet. I can't immagine that the D&RGW would lease a K-28 for the same $'s as a K-27. Comparing my HOn3 K-28 and K-27 models, another aspect comes up, the K-28 has a longer wheelbase, I guess the K-27 "curved" better than a K-28 on the RGS. As others have stated before; the K-28 ran faster and better than other K's. Speed was certainly not a must on the RGS. It's also possible that the K-27 had more "oomph" at slow speeds.
Boris