Thanks Gus for the correction, and the information you provided is much appreciated. Thanks!
on Oct 05
This is an abandoned Oregon Trunk Railroad Tunnel. 'Finishing touches' were completed in 1912 (presumably concrete portals & tunnel sleeving) as the railroad was already running through it by 1910. This portion of the 'OT' abandoned in 1921. The other portal is right around the knob of the hill. Take the 'high' gravel road.