Source: Interstate Commerce Commission, "Statistics Of Railways, " 1887/8, 1888/9, 1913, 1914, 1926, P. C, 1928, P.Lxxviii; 1935, P.S-113.
This NBER data series a02084ca appears on the NBER website in Chapter 2 at http://www.nber.org/databases/macrohistory/contents/chapter02.html.
NBER Indicator: a02084ca
Release: NBER Macrohistory Database
National Bureau of Economic Research, Railroad Expenditures On Maintenance of Way and Structures for United States [A024CAUSA398NNBR], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/A024CAUSA398NNBR/, September 4, 2015.