This branch left the B&O mainline at Weverton, MD, between Brunswick, MD and Harpers Ferry, WV. It is segment #34 in the Maryland section of Nielsen's book Right-of-Way, A Guide To Abandoned Railroads. The branch was initially constructed some time in the late 1800s, and ran north to Hagerstown. The B&O had its own station in town; a replica on the site is now a car wash. The B&O also had a bypass that cut around the southeast side of Hagerstown and connected directly from the branch to the old Western Maryland mainline; this bypass is abandoned also and ran from Security Junction on the B&O to Security on the WM.
Hagerstown still has a lot of rail activity for a small town, including a former WM yard and buildings that are now the Hagerstown Roundhouse Museum. Part of this branch is still in service, between Hagerstown and a state prison called Roxbury in the SPV atlas. South of there, the entire remainder of the branch was abandoned in the 1970s. Towns on this section to the south of Roxbury are Breathedsville, Keedysville, Eakle's Mill, Rohrersville, Beeler's Summit, Gapland, Brownsville, Augusta, Stonebreaker and into Weverton on the line between Washington, DC and Harper's Ferry, WV.
Thanks to Mike Palmer for contributing information about this route.