The South Boundary Trail has limitless potential for various loops. The 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 that incorporate the South Boundary Trail, the most common is to ride the trail from Angel Fire towards Taos. This direction offers more downhill and smoother climbs.
Garcia Park via FR 437 in Valle Escondido is the easiest access point, FR 76 is a rougher drive and leads to two access points, the proper trailhead and a mile and half further FR 153 leads to a road ascending Osha Peak's south ridge. The first climb from either point is rocky and may require some hiking, the second option isn't as steep. Route finding skills are important in this entire region of the Sangres, besides a few signs at some trail junctions there are lots of unmarked turns, old roads, and meadows where the trail isn't well developed.
Garcia Park is near the middle of the SBT, use this point to shuttle the western half of the trail back into Taos, or to ride Heaven on Earth or Mondragon Canyon. For the classic ride in it's entirety, use the access point from FR 76 south of Angel Fire.
There is some climbing from each access point that requires some hike-a-bike for most people. After these sections, the rest of the climbs are pretty easy. The descents are rocky in places but not too technical, very smooth and flowy in others, and the views are great. When you reach Garcia Park, the trail continues a few hundred feet down the road to the west. It winds through some meadows and climbs into the aspens for a little while, then resumes the descent down through the conifer forests. The last few miles are much more technical and steep, so watch out here, you'll know it when you get there. At the bottom, cross the creek and arrive at the El Nogal trailhead by the highway.
This ride is normally done as a shuttle, some have ridden 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.