Marvin Prater, Adam Sparger, Daniel O’Neil, Jr. Railroad Concentration, Market Shares, and Rates(PDF). United States Department of Agriculture. February 2014 [2022-07-10]. (原始内容存档(PDF)于2022-03-24) (英语).
Marvin Prater, Adam Sparger, Daniel O’Neil, Jr. Railroad Concentration, Market Shares, and Rates(PDF). United States Department of Agriculture. February 2014 [2022-07-10]. (原始内容存档(PDF)于2022-03-24) (英语).