The following railroads merged to form CSX Transportation.
- The Seaboard System Railroad merged with Chessie System which consisted of the Baltimore and Ohio Railroad and Chesapeake and Ohio Railway and Western Maryland Railroad to form CSX Transportation July 1, 1986.
- The Chesapeake and Ohio Railway merged into CSX Transportation August 31, 1987.
- The Richmond, Fredericksburg and Potomac Railroad, which was majority-owned by CSX, merged into CSX Transportation in 1991.
- The Pittsburgh and Lake Erie Railroad, of which part had been used by the Baltimore and Ohio Railroad, sold that portion to CSX Transportation in 1991. On September 11, 1992, CSX Transportation bought the rest through the Three Rivers Railway.
- Atlanta and West Point Railroad
- Baltimore and Ohio Chicago Terminal Railroad
- Chessie System
- Clinchfield Railroad
- New York, New Haven and Hartford Railroad merged with Pennsylvania Railroad and New York Central Railroad to form Penn Central
- Western Railway of Alabama
- Pennsylvania Railroad
- New York Central
- Penn Central Transportation Company
- Erie Railroad merged with Delaware, Lackawanna and Western to from Erie Lackawanna
- Delaware, Lackawanna and Western Railroad merged with Erie Railroad to form Erie Lackawanna
- Erie Lackawanna Railway
- Lehigh Valley Railroad
- Reading Company
- Central Railroad of New Jersey
- Lehigh and Hudson River Railway
- Pennsylvania-Reading Seashore Lines
- Monongahela Railroad
- Conrail
- Chicago and Eastern Illinois Railroad Merged into Louisville and Nashville in 1976