This topic has come up before, so do a search on the list for more responses.
Back in 1968 we chased 493 light engine from Antonito to Alamosa and it was doing 35. So, I suspect some trains might have gone 40-45 a few times on this section. Otherwise the railroad was pretty much a slog-it-out line with many steep grades curves and downgrades. Of course all that is what made it so interesting.
I suspect the San Juan had pretty high speeds on the 3-rail to Antonito as well, and a few other stretches across the Alamosa-Durango line. Remember in the old days it ran all the way through in one day. Now it takes about 6 hours just to go from Chama to Antonito(64 miles).
Greg Scholl