The Wyeth entry A23 in Curious Gorge 4 describes an off-trail exploration of a long-lost remnant of the 1916 Scenic Highway,as well as the former grade of the railroad which ran beside the Scenic Hwy.
Below is a photo circa 1952 showing the building of I-84 and both the railroad and Scenic Highway swinging around the curve along the riverside:
Below is a screengrab of the lidar image of the site. You can see the two grades on the curve out into the Columbia immediately west of Shellrock Mtn.
Find the hidden culvert described in Curious Gorge and then walk thru the dry cement culvert. (Should be dry if you’ve read my tips as to when to go).
Then along the RR tracks heading east from Wyeth until you get to the hidden forest which hides the ghost roads: