Compare your postcard to this one (found on an auction site) which is similar in design and it uses the same "mailed from" marking, but it explains it a bit more:

1. "The Inter-State Co." as shown on the "mailed from" marking is the publisher of the postcard. That's why I refer to it as a "marking" rather than a "postal marking" as it would seem the marking was simply a souvenir and has no postal relevance.
2. The reference to D.& R.G.R.R. is the abbreviation for "Denver and Rio Grande Rail Road".