The narrow gauge guys got away with lots of stuff because no officals wanted to go to Chama and see what was up. This was back in the old days of railroading where as long as you didn't get in trouble, you could get away with lots of stuff.
By the 1960's (and today for that matter) the speed limit between Chama and Cumbres was/is 12 MPH.
The track in many parts is capable of much more than that. However the grade is what gets you. I've come off the hill at 18+ and I'll tell you the air brakes don't do a heck of a lot at that speed! 12mph works out to a 1:09 running time. Add to that 8 mph around Windy Point, 8 mph around the curves at MP 336 and 10 mph at Lobato Trestle, and its tough to make it much faster than 1:10 without speeding. I used to let the helpers get away with 1:03-1:05 without saying anything. Besides, we paid people by the hour, going fast cuts your pay!