I am going to guess the answer to my question is October 9, 1967. The limit for doubleheading from Antonito to Cumbres was 1400 tons. The last really big westbound train run on the narrow gauge was operated on that date, according to data compiled by Earl Knoob. It was only 1335 tons, but I'm guessing that was close enough to the limit that they would have cut the helper in at Antonito. But that is just a guess on my part. The last westbound train over 1400 tons was on 10/2. Earl do you have more info?
Earl's data also shows the 497 was last used by the DRGW on 12/21/67 hauling four boxcars of drill mud to Farmington and returning cab hop to Durango. It was stored serviceable after that.
JBWX