How is it that the section from Ridgely through Cumberland to Frostburg survived past 1976 but most of the other sections west of Hagerstown were ripped up?
The answer is the coal "boom" of the late 1970s. When the through trains stopped using the Connellsville line, a strip mine started loading hoppers where the main had been cut just east of Borden Tunnel. This location was about 3 miles west of where the WMSR trackage currently branches off the former WM main (Switch #9) to go into Frostburg. This operation lasted about 3 years, IIRC. The trains came in 30-car cuts, and were winched through the loader one at a time.
There was also the 2 brickyards on the former Cumberland & Pennsylvania (C&P) RR trackage down to MT. Savage, these locals used the double switchback at Switch #9, this was at most 2-3 days/week.
All this traffic dried up by 1983, CSX subsequently sold the WM line to the City of Cumberland after removing the C&P trackage from Switch #9 to MT. Savage. The WM tracks from Switch #9 to Borden Tunnel were removed in 1988 and relaid on the former C&P grade to Frostburg. The relaid 132 lb. rail from the WM main was a real improvement over the 90lb. rails that had been removed back in 1972.
As for the Ridgeley area, Chessie briefly toyed with the idea of using Knobmount Yard as a coal train only facility, but then started using all the available tracks for car storage. This also kept the Ridgeley car shop (future WMSR shop) in operation for a few years, thus making it available for eventual use by WMSR.
After service to the last remaining customer on the Ridgeley trackage ended in 1995, most of the tracks were taken up, except for a wye with short tail tracks, suitable to turn the WMSR engines.