Thanks for the photo. Big camera on a tripod, and slow speed film.
Close to Highway 299 and the Mad River, just east of Arcata, probably Korblex, is an active sawmill that may well have been one of the customers of tha Arcata & Mad River. Next to the mill are several Railbox 40 footers that got left behind the last time the Northwestern Pacific failed. Another active mill, probably Glendale, is visible from the highway a few miles further east.
Mike Walker's CALIFORNIA AND NEVADA shows the area in considerable detail.