Daily Averages Are Used. Source: Interstate Commerce Commission, Freight And Passenger Service Operating Statistics Of Class I Steam Railways In The United States.
This NBER data series m01264a appears on the NBER website in Chapter 1 at http://www.nber.org/databases/macrohistory/contents/chapter01.html.
NBER Indicator: m01264a
Release: NBER Macrohistory Database
National Bureau of Economic Research, Unserviceable Locomotives Assigned To Freight Service, Class I Railroads for United States [M0164AUSM481NNBR], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/M0164AUSM481NNBR, February 14, 2016.