The Northern Central Railroad (NCR) Trail is another outstanding Rails-to-Trails project in the Northern Virginia/Maryland/Washington DC area. http://bikewashington.org/trails/ncr/ncr.htm
Together the NCR trail combined with the York County Heritage (YCH) Trail combine for 41 miles of shaded, scenic, mostly packed gravel trail that winds from Cockeysville, Maryland to York, PA. Most of the trail follows the Gunpowder Falls River, so there are several overlooks and waterfalls along the way.
There are a handful of portapotties along the trail, but not much other civilization. Ensure you bring water, spare tubes, and anything else you might need during your ride.