We did some work out in the hills beyond Cazadero and noticed this as we went by on the way north. The Cazadero road is apparently on the old grade for most of the way (the grade was on the west side of the creek as is the road).
North Pacific Coast map
What is left going north is mostly full of driveways and homes, and the creek moves around a lot. But there is this short section right next to the road as you're heading toward Cazadero just after turning off CA116.
By the way, once you get beyond Cazadero, you can see why the railroad stopped there!
Best wishes,
Timothy