Seymour is a city in Jackson County, Indiana, United States. The population was 17,503 at the 2010 census. Seymour is called the "Crossroads of America" because the north/south and east/west railroads intersect in downtown.The north/south line, the Jeffersonville, Madison and Indianapolis Railroad, was built in the 1840s connecting Indianapolis to the Ohio River at Jeffersonville.