The airline who really has a lot to gain from a regional rail standpoint is Delta. From their Atlanta hub the train even today on unupgraded tracks you could easily eliminate flights to Chattanooga, Columbus, Greenville, Augusta, Valdosta, Birmingham, and Montgomery. Delta being as busy as a hub as it is those slots could be better allocated than they already are. Trains have less staffing required, more fuel efficient, and the ability to add intermediate places to the airline network without needing an airport.