The Mountain Subdivision is a railroad line owned and operated by CSX Transportation in the U.S. states of Maryland and West Virginia. The line runs from Cumberland, Maryland, west to Grafton, West Virginia, along the original Baltimore and Ohio Railroad (B&O) main line. 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.