Post San Juan K-28's

Of course, while it's interesting to contemplate the "what-ifs" of a K-28 being leased to the "Southern" or whether it would track well or whatever, the reality of the situation is that there is just no way in Hell it would have ever happened.
Until January, 1951, the remaining three K-28's were needed for the San Juan. For some short period after that, 473 was used out of Chama to power the abbreviated service between Chama and Dulce to meet terms of the abandonment of the mail contract to the Reservation.
Within months after that, the RGS was history. Soon after that, both 473 and 478 were fitted with Power Reverse for switching service. Only 476 was spared. With their 44" drivers, the 470's could be used in double-headed service with the 480's and 490's. Because of their 44" drivers, it would have been impracticle to put them to use on the RGS anyway.
As was recently discussed here, this was all during the Pearlman era, and clearly his goal was to eliminate as much of the Narrow Gauge as quickly as possible. It's really a miracle that any of the K-27's survived at all.