Data Exclude Switching And Terminal Companies Throughout. Source: Interstate Commerce Commission, Data For 1920-1934: "Freight And Passenger Service Operating Statistics Of Class I Steam Railways In The United States". Data For 1935-1943: "Freight Train Performance Of Class I Steam Railways In The United States".
This NBER data series m03050 appears on the NBER website in Chapter 3 at http://www.nber.org/databases/macrohistory/contents/chapter03.html.
NBER Indicator: m03050
Release: NBER Macrohistory Database
National Bureau of Economic Research, Train-Miles Per Train-Hour, Freight Service, Class I Railroads for United States [M03050USM614NNBR], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/M03050USM614NNBR/, October 7, 2015.