Why did the valley line disappear so fast? Let me offer this thought.
The Alamosa Shops. The Alamosa Shop were able to handle heavy repairs.
Where as the Salida Shop could handle running, light, and perhaps medium repairs. The Valley Line provided access to heavy Shops for Salida and Gunnison. Remember by this time, the only way to get something NG to Denver, was by flatcar. And the handwritting was on the wall, for the remaining NG operations.
In Denvers mind the Valley Line abandonment was key to the abandonment of Marshal Pass and Gunnison. Wasn't about this time frame that the line to Montrose was cut by the mud slide?
Get rid of the Valley Line, and you cut off getting equipment to Alamosa for heavy shop work. The Monarch engines were sent from Salida to Alamosa by Flatcar.
So why did the Valley Line disappear so fast. To set up the abandonment of the Gunnison lines and the standard guaging of the Monarch Branch.
As we all know, if it had not been for the oil boom in Farmington, The Alamosa to Durango, Farmington would have been abondoned by 1960. And folks the Denver & Rio Grande Western Narrow Guage RR would have been something only to read about.
My thoughts keith