Southern trail head in DC off of Beach Dr. at Pierce Mill. Northern Trailhead opposite of parking lot of Beach Drive at DC/MD line.
Windy mostly-natural trail (about 1/2 mile is paved) running through Rock Creek Park on the west side of Rock Creek. Mostly smooth single-track although the northernmost mile has a nice steep grade to run up. One of the best inner-city running trails in the world. Connects with the Valley Trail for a 10 miles loop.