The Boundary Trail is 53.7 miles long and has numerous access points and trail junctions. The trail once formed the boundary between the Columbia and Rainier National Forests. Now it marks the southern boundary of the Cowlitz Valley District. This was an important trail in the early history of the Forest Service as it was the principal route used by rangers on horseback patrols. The trail was well established by 1911.
[The USFS site also has descriptions of several segments of the trail.]
Jameson Clover submitted the route (see her FKT report for many photos):
Starting at the east end of the Boundary Trail at Council Lake CG near the base of Mt. Adams. Boundary Trail #1 runs west, and north of Mt St. Helens. The Boundary Trail ends near Coldwater lake at Hummocks TH. It is approximately 58 miles, 11.3K gain, and 12K loss according to my Garmin track, which was on UltraTrac at times (which is why the track is 63 miles). Web research suggested the trail was 53-58 miles, and had 12.5-15K gain, and more loss. On paper the route doesn't look difficult with the exception of it being in the backcountry and very littlle water along the way. The first 28 miles where on dirt bike trails, which made for difficult running. The trail was was rutted and steep with very few switchbacks, but was thankfully covered for much of the way. After Elks Pass TH the trail turned to runnable single track, but became increasingly exposed. There are only two places to get water. There is a pump at Norway Pass TH, and a creek 2 miles away. This was an issue most of the day. The entire trail offers spectacle views of Adams, Rainer, St. Helens, and many more mountains. Huckleberries and blueberries line the trail.
Me and David Mitchell will be running this trail unsupported on 9/30. We will receive no outside aid and will carry everything we need from start to finish except for water.
Update. We ran the Boundry trail unsupported in a time of 15hrs 14 mins. We carried everything we needed from start to finish except water which we refilled at creeks and a lake.
I will add a gpx track and trip report shortly.