ColNei from South Carolina on 8/14/2018 6:13:30 AM:
August 2018 - Just finished riding Katy from Machens to Boonville on a fully loaded tandem with extra wheel trailer without issue other than a mystery flat tire. Bicycle access to the Machens trailhead is via a dirt field access road - look to your right to see the unmarked footpath to work your way through boulders and the old Katy gate. There is no car access and no safe place to leave a car near the trail. If you are coordinating a drop-off or pick-up for this trailhead, it might be easier to call Portage Des Sioux the meeting spot (church parking lot).