Mr. Dorman's book "Durango Always a RR Town" recently spoke of on this list as last 40 copies are for sale at D&S RR Gift shop, has many old photos showing the Durango yard trackage as dual gauge. The RGS Story Vol. 8 has a great two page spread north view of 1 mile below Durango showing the River, and dual gauge main line.
The oil boom at Farmington was the reason to re-gauge the "Red Apple line and the first tank cars moved to Farmington August 14, 1924. The RGS was involved in the first oil traffic to Salt Lake City. In 1923 the ICC approval the construction of the New Mexico Central (SF&NW) to extend to Farmington but it was not done. This scared the D&RGW to make defensive actions and the line was narrow gauged in 1923 to improve service from Alamosa.
This story will be part of RGS Story X next year.