Content Is "Net Increase Of Mileage Of All Railroads In The U.S. In The Calendar Year," An Increase Being Defined As "First Track Mileage Operated." Source: Poor'S Manual Of Railroads, 1912.
This NBER data series a02082 appears on the NBER website in Chapter 2 at http://www.nber.org/databases/macrohistory/contents/chapter02.html.
NBER Indicator: a02082
Release: NBER Macrohistory Database
National Bureau of Economic Research, Increases In Railroad Mileage for United States [A02082USA374NNBR], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/A02082USA374NNBR/, September 2, 2015.