Technically, all of the Cedar River Trail outside of the City of Renton is open to equestrians, but since most of it is paved, it doesn't make for much fun riding. The unpaved, horse-friendly part of it is the stretch extending from Landsberg Road at the south end up to Maple Valley near Route 169.
Since this trail follows an old railroad grade, it is completely flat. It crosses the river and some roads on old trestles which have newer hard surfaces (and railings!), but is otherwise gravel up to the point where it meets the Green-to-Cedar Rivers Trail -- beyond that the trail is paved the rest of the way into Renton.
Landsberg trailhead parking is on the north side of the Cedar River off of Landsberg Road.
From the west/south: Four Corners (between Maple Valley and Black Diamond, the intersection of Maple Valley Highway 169 and Kent Kangley Road/Highway 516), go east on Kent-Kangley Road and take the first left onto SE Summit Landsberg Road. Where it ends and joins Landsberg Road, turn left and proceed down the hill, cross the river, and the parking is on your left.
From the north: Landsberg Road is also known as Issaquah-Hobart Road/SE 276th. From route 18, go south on this until the road turns and the name changes. The parking area will be on the right side of the road.
Length: 5 miles unpaved (17.3 miles overall)
Surface: hard gravel, side trails dirt. Solid concrete surface on trestles.
Share with: pedestrians, bicycles, strollers. Rollerbladers on paved northern segment.
Amenities: Landsberg Park, across the road from the Landsberg parking area, contains porta-potties and picnic facilities. This area is not horse-accessible.
Parking for: up to 5 trailers
depending on other trail users and how they've parked
King County Parks:
KC Parks - Cedar River Trail page
Appears on King County's Regional Trails map.