sentences:  The rail line originally connected Huntsville to NC&StL's Nashville-to-Chattanooga mainline in Decherd, Tennessee.The depot was once the smallest union station in the United States when it served the NC&StL and Memphis and Charleston Railroad, the predecessor to the Norfolk Southern.The Huntsville International Airport is served by several regional and national carriers, including Delta Air Lines, United Airlines, Frontier Airlines, Silver Airways and American Airlines.