The Tunnel Lake Trail is high alpine singletrack accessed from just outside of the ghost town of St. Elmo or from FS 839 Alpine Tunnel 4x4 Road
near the top. Technically this is part of the CDT (Continental Divide Trail), but this segment in particular has its own name.
The views are fantastic along this primitive trail as it meanders through the high alpine tundra ending in a fast flowy switchback descent.
The trail is very rocky at times above tree line with a few sections of hike-a-bike unless you're a very strong rider in terms of leg strength and lung strength. The descent and switchbacks through the dark timber near the ending has a very well maintained and non-technical nature.