Formed in 1973, the Chessie System consisted mainly of the Baltimore & Ohio Railroad, the Chesapeake & Ohio Railway, and the Western Maryland Railway, along with other subsidiaries.

While the C&O had acquired the B&O in 1962, and the two had jointly controlled WM since 1967, they all operated separately under their own names, with their operating departments slowly getting integrated over the years.

Chessie and Seaboard Coast Line Industries merged in 1980, forming CSX. CSX, however, operated as a "paper railroad" (an entity that exists on paper only, and does not own any trackage, rolling stock, or locomotives) until 1987, leaving the Chessie System to continue operating under the Chessie System name until that time, after which the Chessie name was retired.