Thanks Fritz, Dan and others for the great follow-ups. The spur might have been built in emergency conditions as Silverton could starve, and also under the oriiginal Timber Laws which gave a railroad right to timber (cut ties) "adjacent to their lines". Later what "adjacent" meant was in question. The timber laws reverted to limit the railroad rights, and many timber law suits developed against the railroad interests.
The valuation map probably shows it all on the railroad right of way to indicate total ownership and not industry ownership as most siding would be from the clearance point of the switch.
What is or needs to be done to protect the Sawmill Site as it is likely the last of it kind in Colorado, still having a boiler in place? The cars are lost but will the fire box door and other scrap rail still be laying around for the next visitor? Mark Evan has beautiful photos on his Silverton Branch Link.