This is the description offered by Bike Washington (Http://bikewashington.org/trails/cct/cct.htm)
"The Capital Crescent Trail (CCT) is a rail trail project running though the Northwest D.C. Area. On weekends, it is a VERY popular spot for local cyclists and rollerbladers. During weekdays, bike commuters use the CCT to travel between Downtown DC and the Northwest Suburbs.
The southern 7 miles between Georgetown and Bethesda has a 9-foot wide asphalt surface and is in excellent condition. The northern Georgetown Branch extension provides a crushed gravel surface for the final 4 miles between Bethesda and Silver Spring. The two trails are connected by a brand new tunnel located under downtown Bethesda."