Data Exclude Switching And Terminal Companies From 1907-1909 And 1932-1943. Revised Data Are Used For Monthly Figures, 1907-1960. Thereafter No Revised Monthly Figures Are Shown Because Data Only Published Quarterly From 1962 On. Beginning In 1962, Only Quarterly Data Are Published By The Icc; The Monthly Figures Presented Were Derived By NBER. Source: Interstate Commerce Commision, "Operating Revenues And Operating Expenses Of Class I Steam Railways In The United States", Monthly (Called "Class I Railroads" In The Title After November 1956.)
This NBER data series m03040 appears on the NBER website in Chapter 3 at http://www.nber.org/databases/macrohistory/contents/chapter03.html.
NBER Indicator: m03040
Release: NBER Macrohistory Database
National Bureau of Economic Research, Railway Operating Expenses, Total, Class I Railroads for United States [M03040USM144NNBR], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/M03040USM144NNBR/, November 28, 2015.