"Take the 'A' Train" was composed in 1939, after Ellington offered Strayhorn a job in his organization and gave him money to travel from Pittsburgh to New York City . Ellington wrote directions for Strayhorn to reach his house by subway. The directions began with the words "Take the A Train," referring to the then-new A subway service that runs through New York City, going at that time from eastern Brooklyn , on the Fulton Street Line opened in 1936, up into Harlem and northern Manhattan, using the Eighth Avenue Line in Manhattan which opened in 1932.
Although Strayhorn said he wrote lyrics for it, the recorded first lyrics were composed by, or for, the Delta Rhythm Boys . The lyrics used by the Ellington band were added by Joya Sherrill , who was 20 at the time (1944). She made up the words at her home in Detroit while the song played on the radio. Her father, a noted Detroit activist, set up a meeting with Ellington. Owing to Joya's remarkable poise, singing ability and her unique take on the song, Ellington hired her as a vocalist and adopted her lyrics. The vocalist, who most often performed the song with the Ellington band, was trumpeter Ray Nance who enhanced the lyrics with numerous choruses of scat singing . Nance is also responsible for the trumpet solo on the first recording; it was so well-suited for the song that it has often been duplicated note-for-note by others. - more at Wikipedia