Why 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. The Rio Grande wanted to keep all the revenue for itself.
That being said it doesn't mean that D&RG-C&S interchange didn't happen.
Baldwin coal and Farmington petroleum products needed to be interchanged. The Rio Grande Southern as the bridge carrier on petroleum products destined for the Gunnison country got the raw end of that deal.
If the car was interchanged with the C&S the Grande would lose not only a lot of mileage but also terminal switching fees in Denver.