The Grand Ridge trails extend, with a few branches, from I-90 at the south edge to near the Issaquah-Fall City road on the north side. The trails are accessed from the Issaquah-High Point Trail. The initial climb from the flat Issaquah Trail up onto the Ridge is long and steep (the High Point Trailhead access is steeper, with more switchbacks), but the rest of the trail system is rolling forestland, including stands of large red cedars.
The big drawback for this trail is its parking. The "lot" is just a widened end of the rail-trail, and there is no possibility of turning a trailer around in it, so head-in, or better yet, back-in parking is the only possibility; and two rigs will only fit if other vehicles haven't blocked the way. The parking area seems to be a frequent dumping site, too, so be wary of debris. (This area and section of the Issaquah-High Point Trail is owned by the Department of Transportation, so recreational parking isn't their highest priority.)
From the parking area, cross the bridge (you'll probably want to lead your horse around the gate) and ride west down the old rail-trail, ignoring two narrow paths that go up steep banks on the right. Just over half a mile down the trail, a wider path to the right with a small ditch in front of it marks the High Point Trailhead, with the Coal Mine Trailhead not quite a half mile further on - it has a signpost, but no actual sign at the time I write this.
Note: The D.O.T. is currently working on an extension of the Issaquah-Preston Trail from High Point to Issaquah-Preston Road. It is not clear whether this extension will be open to horses (or even if it's worth it to be).
From I-90, take Exit 20. If heading east, turn left at the bottom of the ramp and pass underneath I-90 and park in the gravel lot to the west (left). If heading west, the parking spot is diagonally to your right across from the bottom of the exit ramp. Back-in parking recommended.
Length: 9.5 miles
Surface: hard large gravel on regional trail, dirt in forested parts; starts with immediate bridge crossing
Share with: hikers, bicycles, joggers
Parking for: 2 trailers if
everyone parks nicely - park facing out to avoid being blocked in
King County Parks has added a downloadable "brochure" (including a map) for the Grand Ridge trails to their "Backcountry Trails Map Brochure Series."