The Sums Of Actual Items Were Corrected For Seasonal For Trunk Lines, Western Lines, Southern Lines And Coal Roads. Source: Arthur A Cole, "A Monthly Index Of Railroad Earnings", Review Of Economic Statistics, February 1936, Pp. 31 And Ff.
This NBER data series m03015a appears on the NBER website in Chapter 3 at http://www.nber.org/databases/macrohistory/contents/chapter03.html.
NBER Indicator: m03015a
Release: NBER Macrohistory Database
National Bureau of Economic Research, Railroad Gross Earnings for United States [M0315AUSM144SNBR], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/M0315AUSM144SNBR, May 2, 2016.