Jeff,
I just got off the phone with Elvin, and he said that they never had any semblence of common carrier status. As a matter of fact he laughed when I asked him and said, "No, we had some much damned trouble getting the logs as it was that we couldn't carry no freight!" He did say that they did haul groceries and gas to the camps though, using Sumpter Valley freight equipment.
As for what year they stopped using the Middle Fork, Elvin wasn't sure as he left in 1941. He did say his Dad worked there until about 1945 however, and was under the impression that it was mostly truck logging by that time. I also talked with a man who helped tear up the Middle Fork, and he seemed to remember it being around 1947 when this happened. (Incidently, he said they used the #104 on scrapping ops). My best guess is that 1943 was the last year any major rail operations happened on the Middle Fork, while the track may have stayed in for several more years.
Hopefully this helps!
Casey