((would the Rio Grande interchange freight at Gunnison, Buena Vista or Leadville with the C&S when it could take freight on a 100 percent D&RGW routing? There would be no need to interchange.))
Yes it could do a 100% routing, but the freight would have to be transloaded from narrow gauge to standard gauge at Salida. Depending on the type of load, it might be cheaper to keep on the narrow gauge and let the C&S share the revenue.
((That being said it doesn't mean that D&RG-C&S interchange didn't happen.)) It did happen and it happened often. There is plenty of documentation and a now deceased friend of mine was agent at Crested Butte in the teens and 20s and told me that he waybilled many C&S cars over the D&RGW and D&RGW cars over the C&S.
Jerry Day