Take the 50 largest transportation companies in the 1980 Fortune Double 500 Directory. Cross off number 11 -- Seaboard Coast Line Industries -- and number 13 -- Chessie System. Make a new entry: number 2 -- CSX Corp. -- just between Trans World Corp., number 1, and UAL Inc., old number 2.

The merger of Chessie abd Seaboard into CSX was formally consummated this month, less than two years after their joint application to the interstate Commerce Commission.

Along with the increased competition that is expected to come from rail deregulation legislation, the merger is just one of many developments that is expected to make the rail industry more dynamic than it has been in decades.

CSX starts life with more than $7.4 billion in assets and $4 billion in annual operating revenues. With new headquarters in Richmond, the parent company will operate two major rail systems -- the Chessie System headquartered in Baltimore and Cleveland and the Family Lines Rail System headquartered in Jacksonville, Fla. The combined rail companies will offer service over more than 27,000 miles of track in 22 states, the District of Columbia and Ontario, Canada, becoming the nation's first unified rail system to extend from the industrial Northeast and many of the industrial centers of the Great Lakes region to the Southeast.

The Chessie's principal rail subsidiaries are the Chesapeake and Ohio, the Baltimore and Ohio and the Western Maryland. Seaboard's Family Lines Systems includes the Seaboard Coast Line, the Louisville and Nashville and the Clinchfield Railroad Co. CSX also acquires control of Richmond, Fredericksburg and Potomac Railroad.

CSX's new 24-member board of directors, which met for the first time last week in Richmond, is comprised of 12 directors each from the former companies. wPrime F. Osborn III, the former chairman of Seaboard, is the new chairman of CSX; Hays T. Watkins, the former chairman and president of Chessie System, is now the president of CSX.

Among the new officers of CSX are Robert L. Hintz, senior vice president -- finance, who had the same title at Chessie System; John W. Snow, senior vice president -- corporate services, formerly v.p. -- government affairs for Chessie System, and who had served in a number of positions at the Department of Transportation including deputy undersecretary and administrator of the National Highway Traffic Safety Administration; and Woodruff M. Price, vice-president -- government relations, who had served previously as special assistant to former DOT Secretary Brock Adams.