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

The line includes the well-known Sand Patch Grade over the Allegheny Mountains.

The Pittsburgh and Connellsville Railroad (P&C) was first incorporated in 1837, but did not succeed in raising money until 1846.

[9] A new bridge over the Youghiogheny River at Sinns opened in 1968, connecting the old P&C to the parallel Pittsburgh and Lake Erie Railroad (which the B&O had trackage rights over to New Castle[4]) on the opposite shore.

In summer 1985, the Chessie System (the B&O's holding company at the time) announced that it would sever its St. Louis line through Ohio, rerouting all through traffic over the Alleghenies at Sand Patch rather than along the older Mountain Subdivision.