Hello Jim
I am not as informed as most who post here, but through recent reading, I may have the info you are looking for.
So, here it goes. It is my understanding that the smelter, being on the other side of the river, was in an area that both the RGS and the D&RGW switched. I think that it was RGS right of way, with the D&RGW having trackage rights.
When the RGS quit operations, the bridge across the Animas River became the D&RGW right of way, along with the track to the smelter over there. This area was once a yard that was used for more than the smelter, I believe that a saw mill once loaded over there too.
As far as the dual guage goes, I don't think that it crossed the river on that bridge, as well as the times for these things may have not occured together, as the dual guage was removed after the Farmington branch went from SG to NG. It is my understanding that the Farmington branch connected to Durango at Carbon Jct, near the area of Walmart today.
This is of coarse based on my own limited knowledge, and would need to be corrected by those that with more than I.
Also, the RGS used, under contract, two stalls in the engin house, as well as some of the trackage in the Durango yard.
Phil