Series Is Presented Here As Two Variables--(1)--Original Data, 1920-1929 (2)--Original Data, 1929-1943 Beginning In 1929, Coverage Includes Motor-Car Train-Miles, Not Previously Reported. Source: Interstate Commerce Commission, Data For 1929-1934: Derived By NBER By Dividing Series (Revenue Passenger-Miles) By Series (Passenger Train Miles). Data For 1935-1943: "Revenue Traffic Statistics Of Class I Steam Railways In The United States."
This NBER data series m03052b appears on the NBER website in Chapter 3 at http://www.nber.org/databases/macrohistory/contents/chapter03.html.
NBER Indicator: m03052b
Release: NBER Macrohistory Database
National Bureau of Economic Research, Passenger-Miles Per Train-Mile, Class I Railroads for United States [M0352BUSM503NNBR], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/M0352BUSM503NNBR/, August 28, 2015.