Back to the Horsetooth Mountain Ride map.
The Soderberg Trail just sort of turns into the Spring Creek Trail. I'm not certain, but I think this happens at a point where the Soderberg Trail (which is wide like a service road) sort of dead ends, and the trail does a right-hand turn and goes down some stairs, set with timbers. At the bottom of the stairs the trail sweeps to the left, goes through a short, techy section, and then crosses the narrow, usually dry Spring Creek. There is a small wooden foot bridge across the creek. A narrow footpath follows the stream bed (the Horsetooth Falls Trail). It isn't worth trying to ride.
Follow the Spring Creek Trail. You'll probably have to carry your bike because there are some steep stairs set with timbers. I have friends who ride those stairs down, but it's pretty technical. You'd have to be magic or motorized to ride up them...
Once you've climbed the half-dozen or so stairs, the trail becomes rideable, but it's pretty techy for a couple of hundred yards. It switches back several times and crosses through some loose, rocky sections. It should only take you a couple of minutes to get through those, then you're into a nice open meadow. I have some nice 24-bit jpegs from that area here. For the next mile or so, the Spring Creek Trail is a classic, mellow singletrack.