In the days before air travel air line was a common term for the shortest distance between two points: a straight line drawn through the air (or on a map), ignoring natural obstacles. Hence, a number of 19th century railroads used air line in their titles to suggest that their routes were shorter than those of competing roads.
The Seaboard never owned an airplane. In 1940 the railroad proposed the creation of “Seaboard Airlines,” but this idea was struck down by the Interstate Commerce Commission as violating federal anti-trust legislation.
During a spate of interest in aviation shares on Wall Street following Charles A. Lindbergh‘s trans-Atlantic flight in 1927, Seaboard Air Line shares actually attracted some investor curiosity because of the name’s aviation-related connotations; only after noticing that Seaboard Air Line was actually a railroad did investors lose interest.
The CSX Abbeville sub (Tucker, Lilburn, etc) is the major remaining Seaboard line in the Atlanta area. The line west of Atlanta became the Silver Comet Trail.