Here’s a photo of the station ID plaque on the 50th Street station on the elevated West End Line on New Utrecht Avenue. In 1949 all BMT trains were numbered, but letters took over in the 1960s. Over the years B and M trains have plied this line, but currently, it’s the D.
The station signs were dark blue enamel with serifed white lettering, in a font designed when this el was first built in 1917. It also turns up in other el lines and subways; fonts used nowhere else found their way into the NYC transit system between 1908 and the 1960s.
All that changed when the Unimark station ID system took over around 1970, and eventually settled on black signs with lettering in Helvetica. The Transit Authority, and later the MTA, has had a passion for replacing signage with the Unimark standards and older signage that is revealed by, say, the removal of an ad or a newspaper stand is quickly removed and replaced by modern design.
Uniformity may be important to the MTA, but Unimark can’t match the charm of these earlier sign designs.
PS. They can slap watermarks on every photo online; I’m still going to use them.