Series Is Presented Here As Two Variables--(1)--Original Data, 1920-1930 (2)--Original Data, 1930-1943 Source: NBER Derived Data By Dividing Series (Passenger Car-Miles) By (Passenger Train-Miles).
This NBER data series m03061a appears on the NBER website in Chapter 3 at http://www.nber.org/databases/macrohistory/contents/chapter03.html.
NBER Indicator: m03061a
Release: NBER Macrohistory Database
National Bureau of Economic Research, Passenger Car-Miles Per Train-Mile, Class I Railroads for United States [M0361AUSM257NNBR], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/M0361AUSM257NNBR/, October 8, 2015.