Best to start with 15 or 7.5 minute USGS topo historic maps. In the mid-60s the line west of Dayton went to Grisdale (trucks to train)in the Wynoochee Valley. North of Dayton it went to the Fir Creek Reload near Camp Govey(trucks to train). Prior to that time frame the line went further N, (of Fir Creek) to Camp 3 and then crossed the High Steel Bridge (google). From Grisdale it also went N but not to far. There was a large wood bridge over the Wynoochee River. When Ring of Fire was filmed (David Jansen) the bridge was burned sending the rescue train down into the river. For most of the life of the RR all logs were sent to the mills in Shelton (location of the round house). The Dayton Mill 5 was a relatively recent addition for some of the saw logs. The green lumber was then hauled to Shelton for further processing.
That's the nut shell version
RB