No answer from this corner. I have wondered as well when the line was first built down the Middle Fk. At the museum/SVRy depot in Prairie City they have quite a bit of SVRy Co. paperwork. I was perusing some years back and saw a letter from a mine at Susanville, or perhaps it was a copy of the letter sent by SVRy to the mine mgmt. The mine had requested that the SVRy build a spur to their operation. This was pretty early, likely before the Prairie extension was built or around that time. SVRy suggested that they could do this if the mine would pay for the extension. So far as we know this extension never happened. We know that the OLC line was rebuilt around the same time in the late 30's that SVRy replaced all the rail on the mainline, installed treated ties and new ballast.