There is a common belief that Helvetica is the signage typeface of the New York City subway system. But it is not true - or rather, it is only somewhat true. Helvetica is the official typeface of the MTA today, but it was not the typeface specified by Unimark International when they created the signage system at the end of the 1960s. Why was Helvetica not chosen originally? what was chosen in its place? why is Helvetica now used? when did the changeover occur? Paul Shaw answers these questions in his book Helvetica and the New York City Subway System: The True (Maybe) Story.
Find more info about the book here.