If you want The (Mostly) True Story of Helvetica and the New York City Subway, AIGA has it for you in a sprawling (and well-worth-reading) 9-page article on New York's subway fonts. They state that while it's commonly believed that Helvetica is the official typeface of the New York City subway system, that hasn't always been the case (of course, since the font just celebrated its 50th anniversary, and the first underground transit system in New York City was built in the late 1800s). While "Helvetica is the official typeface of the MTA today, it was not the typeface specified by Unimark International when it created a new signage system at the end of the 1960s." Trace the typeface of the underground, font nerds, you know you want to!