Source: Hickman, "The Volume Of Corporate Bond Financing Since 1900, " (NBER) Pp. 312-313.
This NBER data series q10082 appears on the NBER website in Chapter 10 at http://www.nber.org/databases/macrohistory/contents/chapter10.html.
NBER Indicator: q10082
Release: NBER Macrohistory Database
National Bureau of Economic Research, Net Credit Change, Bonds, Par Value, Railroads for United States [Q10082USQ311NNBR], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/Q10082USQ311NNBR/, October 9, 2015.