The Mountain Subdivision is a railroad line owned and operated by CSX Transportation in the U.S. states of Maryland and West Virginia.

It was known as the West End Subdivision until the B&O's absorption into the Chessie System, and included the B&O's original crossing of the Allegheny Mountains.

Through CSX traffic to the west from Cumberland now uses the Keystone Subdivision over Sand Patch Grade.

[6] In 1904 the B&O built the Patterson Creek Cutoff to alleviate congestion in its Cumberland rail yard.

[7] During the 1970s ownership of the subdivision passed from the B&O to the Chessie System, which closed the Patterson Creek Cutoff.