4Henry Varnum Poor. History of the Railroads and Canals of the United States of America. New York: John H. Schulz & Co., 1860.
5Standard & Poor's. 'AAA/A-I+' Rating O11 United States of America Affirmed, Outlook Revised To Negative http://www.standardandpoors.com/ratings/articles/en/ ap/?assetID= 1245302886884, 2011-4-18.