These are segments #16 and #17 in the Maryland section of Nielsen's book: Right-of-Way: A Guide To Abandoned Railroads.
The following info is from a brochure from the Walkersville Southern RR, which operates a tourist train http://www.wsrr.org on this line May-October each year:
This branch of the Pennsylvania RR was initially built from York and Hanover, PA through Taneytown and Walkersville to Frederick, MD in 1869. The line served local farming communities and remained in business after the Penn Central merger in 1968.
In 1972, Hurricane Agnes hit the northeast and damaged much property, including the bridge over the Monocacy River, 2 miles south of Walkersville (4 miles north of Frederick). The entire line from Pennsylvania through Maryland was put up for sale by PC at this time, and the state of Maryland bought the segment within its borders. North of Walkersville, the line continued to operate (to this day) as the Maryland Midland.
South of Walkersville the line remained dormant for 20 years, until the Walkersville Southern was formed and started running excursions on the short segment of line. The bridge over the Monocacy River was rebuilt in 1995-1996, and excursions now extend south from "downtown" Walkersville to just north of MD route 26, near a place called 'Harmony Grove' in the SPV atlas.
The southern abandoned section of line is about 2 miles long in and around Frederick. The tracks are removed from the MD route 26 crossing (the entire road has been re-graded), but south of there they are still place. All grade crossing signals are gone and the flangeways are filled in but the tracks are there, parallel to East Street for some distance, with a portion of street running in the city of Frederick. There are some old freight spurs along there, too.
The tracks end at the corner of East Street and East Patrick Street (MD Route 144) at the southeast corner of downtown. About 1/4 mile south of that point is a "new" Frederick passenger station, as MARC (Maryland Rail Commuter) service was re-established to Frederick in 2001. This track to the station is an old B&O spur.
Topo maps show there used to be a bunch of interchange freight tracks near the corners of East Street and South Street, but these were all removed before 2001, though some of the old industries still have warehouses there.
The northern section of the abandonment runs from Taneytown to Kingsdale, then into Pennsylvania and Littlestown, Sell and into Hanover. The section of this former PRR line from Hanover to York was later operated by the Maryland & Pennsylvania (aka the "Ma & Pa"). In Hanover, a short segment of track remains to serve some warehouses. Between Hanover and Littlestown some track segments remain, but none remain in Littlestown.
Thanks to Mike Palmer for contributing information about this route.