From their website:
"From Willits north to Humboldt Bay, the rail corridor has been “railbanked,” allowing the Great Redwood Trail to be built as a rail-to-trail, where the trail will be located within the historic rail alignment. The right of way will be maintained in an unbroken fashion to protect the long-term status of the trail under federal law. From Willits south to the Sonoma County border, the interaction between the railroad and the trail is still being determined. The Great Redwood Trail in Sonoma and Marin County will be rail-with-trail operated by SMART."
If you truly have time on your hands you may inspect their website:
https://greatredwoodtrailplan.org/