Is there a good web site detailing the system of nomenclature/map representation of the NYC subway? Specifically I'm trying to find out when the current convention of letters and numbers began. I'm not all that interested in which lines followed which routes when or what their particular letters were, just the naming conventions themselves.
The best (and most comprehensive) site on the NYC Subway is www.nycsubway.org.
As for the current system of letters and numbers, the lettering for the IND lines dates back to the opening from 1932, the BMT letters from the introduction of the R27/30 cars in 1960, IRT circa 1948.
The route colors have change over the years, the current ones date from the 70s.