Series Is Presented Here As Two Variables--(1)--Original Data, 1921-1937 (2)--Original Data, 1937-1943 Data Include Switching And Terminal Companies. Data Are Midmonth Figures Computed From End-Of-Month Data. Source: Interstate Commerce Commission, Motive Power And Car Equipment Of Class I Steam Railways In The U.S.
This NBER data series m01258b appears on the NBER website in Chapter 1 at http://www.nber.org/databases/macrohistory/contents/chapter01.html.
NBER Indicator: m01258b
Release: NBER Macrohistory Database
National Bureau of Economic Research, Serviceable Passenger Locomotives for United States [M0158BUSM365NNBR], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/M0158BUSM365NNBR/, October 9, 2015.