The 76, along with sister 75, ended up in Peru. To my knowledge, only speculation has surfaced considering their final disposal. It is the 621, a standard-gauge 2-8-0 that operated on the High Line after the line was broadened, which is on display in Leadville.
The answer as to why the 74 survived lies probably in the fact that the Leadville operations wanted a (slightly) more powerful and matching set of engines: the 75 & 76. Plus, the 74 was what the RGS had available for purchase, could afford and was simulataneously light enough for their rail conditions and powerful enough for their motive power needs.
One other issue was the operability of motive power, including the 60. The story is that the crown sheet is shot from burning old rubber tires in the firebox without water in the boiler for smoke effects while on display. This may be an old story or the truth.
Just in passing, I saw the 71 during the brief time she was operating again in Central. What a graceful, downright beautiful creature in motion!