Data Beginning June 1955 Are Preliminary. Source: Interstate Commerce Commission, Revenue Traffic Statistics Of Class I Steam Railways.
This NBER data series m03012c appears on the NBER website in Chapter 3 at http://www.nber.org/databases/macrohistory/contents/chapter03.html.
NBER Indicator: m03012c
Release: NBER Macrohistory Database
National Bureau of Economic Research, Railroad Revenue Passenger-Miles, Commutation, Class I Railroads for United States [M0312CUSM429NNBR], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/M0312CUSM429NNBR/, June 2, 2015.