Wade,
Since you said "Narrow Gauge" and not just the D&RGW, that widens things out a bit. The general consensus world speed record holder for "Narrow Gauge" i.e. less than 56.5" is the South African class 16e 4-6-2, running on a track gauge of 3'6". Since these engines had 72" drivers, poppet valves and ran on well maintained track, they had a theoretical top speed of well over 100 mph. Anecdotally, 100mph + speeds have been claimed, but the railroad's official claim is 86mph. Harold Edmonson, in his book "World Steam in Action" reports 92mph on dynamometer test, as reported to him by an SAR engineman. Anyway you cut it, these babies were fast, but they aren't what we usually think of when we think "Narrow Gauge."
The Sandy River & Rangely Lakes 2 ft. ga. Forney's have been reported to have done "a mile a minute" and rode well in the process. Since the Forney design is exceptionally stable, there may be some truth to this. Using the generally accepted standard of 540 rpm to calculate the top speed, I come up with 55mph, using 35" drivers. Since the Forney is so stable and with a small engine, the motion parts are lighter, it might be possible to exceed the 540 rpm limit.
Trackwork and engine stability are very important. Outside frame engines would have less lateral stability than inside frame engines, since the reciprocating mass is so far outside the track line. I would expect the SV and EBT inside frame engines to be more stable than the D&RGW engines. I ran alongside the 497, coming back into Durango years ago, and she hunted something fierce. They were running at about 25 mph by my speedo. I talked to the conductor later and he said that he had been about to pull the air, that anything over 20mph got really rough with that baby. The K-28's were much smoother and easier riding. Maybe John Coker can let us know what a K-28 could really do!
