Jazz song originally written in 1941 by Billy Strayhorn for Duke Ellington; in time it became the theme song for his orchestra and remains a popular big band standard even today. In an interview with writer Stanley Dance, published in 1970, Strayhorn explained what the terse lyrics were actually talking about:
"The reason we gave it that title was because they were building the Sixth Avenue subway at that time, and they then tuned off and went to the Bronx, but the "A" train kept straight on up to 200 and Something Street. People got confused. They'd take the "D" train and it would go the Harlem and 145th Street, but the next stop would be on Eighth Avenue under the Polo Grounds and the one after that would be in the Bronx. So I said I was writing directions--take the "A" train to Sugar Hill. The "D" Train was really messing up everybody."
Sugar Hill was the nickname for the (at the time) wealthy area of Harlem. It was "sweet and expensive," the peak of African-American culture at the time. The Savoy Ballroom, the Cotton Club, all the best places to go and experience Harlem were there. All you had to do was take the right train.
You must take the "A" train
To go to Sugar Hill way up in Harlem
If you miss the "A" train
You'll find you missed the quickest way to Harlem
Hurry, get on, now it's coming
Listen to those rails a-humming
All aboard, get on the "A" train
Soon you will be on Sugar Hill in Harlem