This was probably a ranch road cut through oak woodlands leading from Alhambra Creek up to the Briones Crest Trail and conveniently called a "trail" when the land was purchased by the park district. Descending from Briones Crest, there's a nice but short section of smooth downhill with views of Mt. Diablo. Then the trail makes a sharp left and drops into a steep gully that is loose dirt when dry or slippery mud when wet. At the bottom is a rocky drop that isn't visible from the top so take a close look before descending.
Spengler didn't like bikers. Or hikers for that matter. There are more gullies and unnecessarily steep pitches to follow and some would be too slippery for horses. This is more of an ATV trail that doesn't seem to follow any natural contours. The many "use" trails in the area show better potential.