The South Boundary Trail begins at the El Nogal Picnic Area on US Highway 64 (~3 miles east of Taos) and ends on FR 76 close to Angel Fire.
There are a few popular rides and several burly loops that incorporate the South Boundary Trail—though most people ride it from east to west, either starting at Garcia Park (40 minute shuttle from Taos) or doing the full ride from FR 76 near Angelfire (1 hr shuttle). This direction offers more downhill and smoother climbs.
The "full" SBT starts from FR 76—a washed-out, rocky access road best tackled in someone else's vehicle—and begins with an 1.5 mile, 800 ft rocky loose climb that may require some hiking. From Osha Mountain, the trail descends a loose road for less than a mile before entering the "Heaven on Earth" section: a consistent flowy classic side-hill singletrack descent through conifer forest and aspen groves. Then wipe the grin off your face with the 1.5 mile, 350 ft climb to Garcia Park.
At Garcia Park, the trail follows the road briefly before cutting right across the field and back into the forest where a series of old two-track and singletrack connectors weave westward. Route finding skills are important there are lots of unmarked turns, old roads, and meadows where the trail isn't well developed. The trail eventually continues it's side-hill traverse / descent through aspen and conifer forests that are even more stunning in the fall.
SBT loses most of its elevation in the last 5 miles, with the steepest and most technical bits thrown at you in the last 3, known as the El Nogal section. This challenging final descent is fast at times, loose at times, stair-steppy at times, scary at times, and exceedingly fun (at times.) Many riders will feel more comfortable walking through the tricky bits.
(For an alternate route, turn south on the Ojitos Trail #166
just past the obvious log drop. It's a longer, but mellower descent to the highway.)
This ride is normally done as a shuttle; some ride it as an out-and-back from Taos in a long day. There is no potable water on the trail, so bring plenty, and watch out for afternoon thunderstorms during summer.