I live near the 96th Street stop on the B/C line and every time I walk by, I can't help but notice that the entrance on the north side of the 97th has a sign indicating that the station is for the B, C, and K train. As far as I know there was never a K train, and everyone I know is equally stumped as to why there's such a sign. Is it just some sort of prank, or did a K train exist way back when?
Although there have been various K trains since 1906, including one that ran from Canarsie, the sign that you see was part of a train that ran along much of the present day C line as recently as 20 years ago. Ask Gothamist is not sure if that qualifies as "way back when."
According to an old MTA brochure, express trains (the A, for example) used to be indicated by single letters and double letters indicated trains making local stops (the AA). In the early 80's, the MTA began changing this system and used only single letters to simplify the subway map and reflect the fact that a train's local or express status was not a line-wide condition, but depended on the time of day or geographic location.
The A train, of course, is still around, so what happened to the AA, its local cousin? Reflecting their new policy of only using single letters, the AA became the K and ran between 168th Street and the World Trade Center. In the mid 80's, the K train was eliminated entirely since the C train covered much of the same ground.
That's not to say that the K train couldn't make a comeback since there are so few letters to choose from for new lines. Gothamist reported that the Second Avenue Subway would be called the T line. One letter, I, will never be used because it looks too much like a 1 on digital display boards.