One of my favorite trails. There are multiple trails that feed into the Highline. Gibralter Ridge Trail #335 off of Grave Creek Trail (9.5 miles), Sinclair Creek Trail #87 and Black Tail Trail #92 off Burma Road are three of the more popular trails that connect with the Highline. As of last summer, trail condition was very good with moderate grades for most of the trail.
There are a couple of short very steep stretches, and the trail crosses a talus slope that is very steep. The views from this part of the trail are wonderful views of big and little Therrault lakes. There is an unimproved camping area at Blue Bird Lake (Note: this is prime grizzly country) and the forest service has provided "goal posts" to hang your food the required 10 feet. You'll need to find the marker that gives the lat/long for this or spend some time searching for it.
This is a mid-to-late summer run (depending on snow accumulation).