I have the the Middle Fork John Day River OLC "main line" and many of the logging spurs entered into a GIS database and have printed off a few maps. There are over 300 miles of grades identified so far. I estimate that it is about 90% accurate, some grades we will never know for sure as they are covered by roads or other changes.
The OLC main line turned up Camp Ck and did not extend further west as far as I can determine. I surmise the Susanville area was logged off early to provide lumber and mine props for the mining activity there so no trees for OLC to cut in 1920 & 30s.
Grades went past Jungle creek as far as Bear Ck and also up Cottonwood Ck to the south. Didn't extend up Camp Ck much past Cougar Ck. The 1939 aerial photos show a logging camp about halfway up Cotton wood Ck with cars on spurs at the camp. The area is now a summer cow camp.
I would like to do the same for the Powder and Burnt River Valleys, but will need help from the WWNF.