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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 Seaboard Coast Line Railroad merged with the Louisville and Nashville Railroad and others to form the Seaboard System Railroad December 29, 1982.
- The Atlantic Coast Line Railroad merged with the Seaboard Air Line Railroad to form the Seaboard Coast Line Railroad July 1, 1967.
- The Charleston and Western Carolina Railway merged into the Atlantic Coast Line Railroad December 31, 1959.
- The Seaboard Air Line Railroad merged with the Atlantic Coast Line Railroad to form the Seaboard Coast Line Railroad July 1, 1967.
- The Piedmont and Northern Railway merged into the Seaboard Coast Line Railroad July 1, 1969.
- The Atlantic Coast Line Railroad merged with the Seaboard Air Line Railroad to form the Seaboard Coast Line Railroad July 1, 1967.
- The Louisville and Nashville Railroad merged with the Seaboard Coast Line Railroad and others to form the Seaboard System Railroad December 29, 1982.
- The Nashville, Chattanooga and St. Louis Railway merged into the Louisville and Nashville Railroad August 30, 1957.
- The Monon Railroad merged into the Louisville and Nashville Railroad July 31, 1971.
- The Georgia Railroad merged into the Seaboard System Railroad in 1983.
- The Seaboard Coast Line Railroad merged with the Louisville and Nashville Railroad and others to form the Seaboard System Railroad December 29, 1982.
- The Chesapeake and Ohio Railway merged into CSX Transportation August 31, 1987.
- The Baltimore and Ohio Railroad merged into the CSX Transportation August 31, 1987.
- The Western Maryland Railway merged with others to form CSX July 1, 1986.
- The Pere Marquette Railway merged into the Chesapeake and Ohio Railway June 6, 1947.
- The Baltimore and Ohio Railroad merged into the 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
<imagemap> Image:Railroadtree.svg|thumb|center|700px|A family tree of Conrail, CSX Transportation, and Norfolk Southern Railway. Links are clickable. rect 63 654 169 694 CSX Transportation rect 457 657 566 693 Conrail Shared Assets Operations rect 783 655 895 691 Norfolk Southern Railway rect 806 551 871 590 Southern Railway rect 482 551 533 567 Conrail rect 369 465 463 501 Lehigh Valley Railroad rect 565 460 676 494 Erie Lackawanna Railway rect 61 435 160 470 Seaboard Coast Line rect 736 400 823 419 Erie Railroad rect 363 379 463 414 Central Railroad of New Jersey rect 562 315 668 366 Delaware, Lackawanna and Western Railroad rect 786 298 894 336 Norfolk & Western Railway rect 379 293 439 328 Reading Railroad rect 58 302 160 340 Seaboard Airline Railroad rect 272 220 329 261 Chessie System rect 592 214 669 257 Penn Central Transportation Company rect 774 198 882 217 Wabash Railroad rect 41 162 150 198 Louisville & Nashville Railroad rect 346 131 439 165 Western Maryland Railway rect 513 102 556 150 New York Central rect 669 101 725 148 Pennsylvania Railroad rect 772 106 886 155 Nickel Plate Railroad rect 798 29 860 67 Virginian Railway rect 600 18 651 74 New York, New Haven & Hartford Railroad rect 436 31 534 64 Baltimore and Ohio Railroad rect 270 31 366 66 Chesapeake & Ohio Railway rect 113 31 205 69 Atlantic Coast Line Railroad
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