Not a widely known trail, and definitely off the radar for the area, but ridden enough by in-the-know locals that the singletrack is well worn. All told, this trail needs to get some recognition. I'm not sure who has been taking care of it, but they've done a nice job.
Heading south from Goya Road on Tortilla Trail, the trail head will be a hundred feet down on the left. It isn't far from the northern most point on Tortilla. You'll have to ford the river/stream to get across and once across it is a quick jaunt to the split. Left or right, you'll end up at this spot again, but I recommend going left.
Left will bring you down the side of the loop that parallels 270S. It is full of twists, turns, switchbacks, fallen trees to hop over, at grade stream crossings and a host of other fun things. The beginning is a hellish uphill section that leave the legs burning for a break. Closer to the end, you'll start seeing the campground sites which may or may not be used. Ultimately, the singetrack puts you out on the dirt access road. Turn right and follow this for a while.
You'll come around a right hand bend and see a bridge on the left side, this is the start of the other portion of the trail. Here is where it gets interesting. Some fallen trees from storms in 2012 force you to hop off the bike and lift it over, and some other sections are just dicey and tough. Thumbs up to you if you can navigate it without any issues. You'll eventually work your way back to the split location, hang a left and head back to Tortilla Trail.
Someone has done an excellent job of keeping this unknown trail up to date and open. Trees have been cut up, new paths cut, paths swept of fallen leaves and generally everything is up to date and ready to be ridden. Next time you're in the Potomac/Bethesda area or at Cabin John Trails, give this little trial a look.