Data For 1907-October 1910 And 1931-1942 Exclude Switching And Terminal Companies. The Figure For June 1918 Was Affected By An Increase In Wages; The Figure For August 1920 Was Also Affected By A Wage Increase, Effective July 20 And Retroactive To May 1, 1920. Source: Interstate Commerce Commission, Data For 1907-1909: "Bulletin Of Revenues And Expenses Of Steam Roads In The United States", No. 5, P. 13; No. 18, P. 13; No. 31, P. 13. Data For 1910-1921: "Statistics Of Railways", 1921, P. C. Data For 1922-1942: Operating Revenues And Operating Expenses Of Class I Steam Railways In The United States".
This NBER data series m03039 appears on the NBER website in Chapter 3 at http://www.nber.org/databases/macrohistory/contents/chapter03.html.
NBER Indicator: m03039
Release: NBER Macrohistory Database
National Bureau of Economic Research, Operating Expenses, Maintenance and Equipment, Class I Railroads for United States [M03039USM144NNBR], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/M03039USM144NNBR/, November 30, 2015.