Data Refer To Baa Grade Bonds. Source: Moody'S Transportation Manual, 1965; Moody'S Bond Survey, Weekly Issues Thereafter.
This NBER data series m13062 appears on the NBER website in Chapter 13 at http://www.nber.org/databases/macrohistory/contents/chapter13.html.
NBER Indicator: m13062
Release: NBER Macrohistory Database
National Bureau of Economic Research, Yields On Railroad Bonds, Lowest Rating for United States [M13062USM156NNBR], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/M13062USM156NNBR/, June 30, 2015.