As merger plans formulated, WM could see its traffic disappear.  The planned merger of the New York Central RR (NYC) and the PRR (the ill-fated Penn Central) could throw traffic from the Pittsburgh and Lake Erie RR (part of the NYC system) onto the PRR. The Norfolk & Western Railway (N&W) could easily reroute traffic from the P&WV onto N&W lines right to Hagerstown.  The WM decided to forsake independence and join the B&O-Chesapeake and Ohio Railway (C&O) — B&O was almost half owner of WM.  B&O and C&O applied to control WM, and the ICC approved their bid in 1967.

There was little evidence of the C&O-B&O control until 1973, when the Chessie System was incorporated to own C&O, B&O, and WM.  In 1973, WM applied to abandon 125 miles of main line from Hancock MD to Connellsville PA.  WM's single track paralleled B&O's double-track line and had easier grades and better clearances, but the expense of maintaning the line and building connecting lines outweighed any savings that might result in lower operating costs.  That same year, WM's Port Covington coal terminal was abandoned in favor of B&O's newer pier in Baltimore.  Gradually, B&O absorbed WM's operations, and in late 1983, B&O officially merged the WM.  The B&O itself merged with the C&O in 1987, which itself became part of CSX Transportation.