This trail doubles as a maintenance perimeter road for the Lake Youngs Reservoir, which is fenced and no entry is permitted. The trail skirts around the outside of the fence, with varied scenery on the "outside" side (from suburban backyards to forested wetlands) and untouched woodland on the "inside". Aside from a few narrow sections, the trail is firm, wide, and relatively mud-free year-round.
From the southwest end of the parking area, a spur trail of a few dozen yards leads to the loop road around the reservoir. The steepest hill on the trail is immediately to your right at this junction, so if you go clockwise you can leave climbing it for the end of your ride, or if you go counterclockwise you can descend it first. There are occasional mileage signs along the way that assume a counterclockwise circuit.
For a longer ride, a ½ mile road-and-right-of-way ride on 216th at the southwest corner of the loop connects this trail to the Soos Creek trail. Cross 148th and follow 216th until it dead-ends, then continue on the fenced right-of-way straight ahead. This drops down a steep switchbacked hillside to pick up 216th again at the bottom, which then intersects with the Soos Creek Trail in a few hundred yards.
Exit 4 from I-405 to Rte. 169 (Maple Valley Highway). Turn right at 140th Way SE, then left at SE Petrovitsky Road. In about 1.5 miles, turn right onto SE Old Petrovitsky Road (Old Petr. Rd. is a loop, so if you miss this turn there is another opportunity across from 184th). The parking area is about a quarter mile ahead on the right.
Length: 9.2 miles (loop)
Surface: gravel, dirt
Share with: joggers, bicycles, walkers, dogs
Restrooms, manure bin, tie rails
Parking for: 4-6 trailers
King County Parks, but they have no separate website for this trail.
Map: K.C.'s Bicycle Guide Map shows both Lake Youngs
Trail but you have to zoom in to find it:
http://www.kingcounty.gov/transportation/kcdot/Roads/Bicycling.aspx Since the trail itself is one single long loop following the reservoir's chain-link fence, I guess nobody figured a trail map was necessary. A few individuals have put up maps on various websites, though, like this one or at the bottom of this one here.