The DSP&P line to Gunnison CO lasted into the C&S era, being abandoned in 1910. So if your modeling pre-1910 it would be possible.
Jeff
But remember, it's your railroad. It all depends on how prototypical you want to be. I'm sure you can come up with an interchange point on some mythical branch - or even dual gauge a stretch of the Rio Grande to be interchanged. Now, I'd like to hear the story about how an East Broad Top or White Pass and Yukon boxcar is sitting in the yard at Durango...
Thanks Ray
Correct, you wouldn't see D&RG narrow gauge cars on the narrow gauge C&S or C&S on the D&RG.
You might have seen DS&P or CC cars on the D&RG and D&RG on the CC or DS&P. I say might because I don't recall ever seeing a picture of interchanged cars.
Ray
So you wouldn't ones frieght cars on the other line?
It's unlikely. The D&RG mainline from Denver to Pueblo was converted to standard gauge in 1890. The narrow gauge Colorado & Southern wasn't created from the Colorado Central and the Denver South Park and Pacific until 1898. It's conceivable that the D&RG interchanged with both of the C&S predecessors since they were parallel from Denver down to Littleton.
Did the DGR&W and the C&S narrow gauge interchange cars?