Hi, Greg -
I rode three 638 trips in 1960-62 while attending college in Colo. Spgs.
The first was in the autumn of 1960, west of Denver to the RR Museum in Golden. There was a runby east of Golden, and the engine ran around the train & ran tender-first back to Denver. The foliage in your shots would be VERY autumn-ish - all the cottonwoods, etc., were in full fall colors.
The second was in late spring of '61 or '62 - out and back on two or three branches to the southwest and then southeast of downtown Denver. Foliage would have been very bright green for the most part.
The third was December 16, 1962 - the last run of the 638 - from Denver down to Trinidad where she was to be stuffed & mounted. Bright winter day, with some patches of snow on the ground and bare trees. There was no tender-first running and, if I remember right, the train would have been longer than the one you describe.
I have a roll or two of slides from each trip, but my new slide scanner is still on back order ...
- El Russo Loco