I didn't notice the D in the corner until I read about this series by Korean artist Yumi Heo on Forgotten New York (http://forgotten-ny.com).

D here stands for Dragon Boat, famous in Queens on account of the Hong Kong Dragon Boat Festival http://www.hkdbf-ny.org/

The entire alphabet runs up and down the Queens Boulevard viaduct for the IRT 7 Train.

For more images, visit: http://www.nycsubway.org/perl/artwork_show?47