Source: Data Computed By NBER From Seasonally Adjusted Data For Series (Railroad Freight Tons Originated, Less Than Carload) Subtracted From Seasonally Adjusted Data For Series (Revenue Tons Originated,Class I Steam Railways)
This NBER data series q03068 appears on the NBER website in Chapter 3 at http://www.nber.org/databases/macrohistory/contents/chapter03.html.
NBER Indicator: q03068
Release: NBER Macrohistory Database
National Bureau of Economic Research, Railroad Freight Tons Originated, Carload for United States [Q03068USQ455SNBR], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/Q03068USQ455SNBR/, August 2, 2015.