Skyline is an unblazed trail that runs through Kettlebail State Forest, a sliver of forest between Morgan Hill and Heiberg Memorial forests. As far as I know, it's the only foot trail through Kettlebail, the only other trail of any kind there being a snowmobile trail that runs with the Skyline Trail for a short distance. I say unblazed, not unmarked, because there are markings along the trail in the form of small stone cairns.
Because I know of no other place online to find directions to the Skyline trailhead I'll post them here. Follow these directions at your own risk. Begin at the parking area just west of Labrador Pond and walk south along the road until the power line crosses the road. At that post go across the road into the underbrush and it'll open up into a trail along a stream. I'll let you figure out the rest. If you follow the trail to its end you will reach Kettlebail Rd and can, from there, make your way to Heiberg Forest by a short road walk.