Data Are For Non-Revenue And Revenue Freight. Data For 1920-1943 Exclude Switching And Terminal Companies. Beginning July 1929, Reports Are Titled "Net Ton-Miles Per Loaded Car-Mile." Data, However, Are Comparable Throughout. Source: Data For 1918-January 1920: American Railway Association Annual Bulletin, 1922, P. 9. Data For 1920-1934: Interstate Commerce Commision, "Freight And Passenger Service Operating Statistics Of Class I Steam Railways In The United States". Data For 1935-1943: Icc, "Freight Train Performance Of Class I Steam Railways In The United States".
This NBER data series m03002a appears on the NBER website in Chapter 3 at http://www.nber.org/databases/macrohistory/contents/chapter03.html.
NBER Indicator: m03002a
Release: NBER Macrohistory Database
National Bureau of Economic Research, Average Load Per Freight Car, Class I Steam Railroads for United States [M0302AUSM525NNBR], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/M0302AUSM525NNBR/, October 6, 2015.