This 19-page US Forest Service booklet provides details, including a map and mile-by-mile description of the trail:  https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5346991.pdf

https://www.fs.usda.gov/recarea/giffordpinchot/recarea/?recid=31494

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.