Data Exclude Switching And Terminal Companies. Data For 1920-1934 Include Light Trains; Data For 1935-1942 Exclude Light Trains. Source: Interstate Commerce Commission, Data For 1920-1934: Computed By NBER From Series (Freight Car-Miles) And (Freight Train-Miles). Data Fir 1935-1942: "Freight Train Performance Of Class I Steam Railways In The United States".
This NBER data series m03058 appears on the NBER website in Chapter 3 at http://www.nber.org/databases/macrohistory/contents/chapter03.html.
NBER Indicator: m03058
Release: NBER Macrohistory Database
National Bureau of Economic Research, Loaded Freight Car-Miles Per Train-Mile for United States [M03058USM256NNBR], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/M03058USM256NNBR, February 7, 2016.