Data Were Obtained By NBER By Dividing Series 03001 (Railroad Revenue And Non-Revenue Freight Ton-Miles) By Series 03045 (Freight Train-Miles). Non-Revenue Ton-Miles Are Included In The Data For Series 03045. Data Exclude Switching And Terminal Companies Throughout. Source: Interstate Commerce Commision, 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 m03046 appears on the NBER website in Chapter 3 at http://www.nber.org/databases/macrohistory/contents/chapter03.html.
NBER Indicator: m03046
Release: NBER Macrohistory Database
National Bureau of Economic Research, Net Ton-Miles Per Train-Mile, Class I Railroads for United States [M03046USM611NNBR], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/M03046USM611NNBR/, July 2, 2015.