Source: Transit Journal, January 1936, P. 6 And Previous Issues.
This NBER data series a03012a appears on the NBER website in Chapter 3 at http://www.nber.org/databases/macrohistory/contents/chapter03.html.
NBER Indicator: a03012a
Release: NBER Macrohistory Database
National Bureau of Economic Research, Total Transit Passengers for United States [A0312AUSA430NNBR], retrieved from FRED, Federal Reserve Bank of St. Louis https://research.stlouisfed.org/fred2/series/A0312AUSA430NNBR/, July 29, 2015.