Duke Ellington

Edward Kennedy 'Duke' Ellington was a composer, pianist and bandleader from America.  His music lets many different musicians express themselves at the same time.  

Questions

How does the music follow the words of 'Take the A train'?

"Hurry hurry hurry take the 'A' train

To get to Sugar Hill 'way up in Harlem

If you should take the 'A' train

You'll get to where you're going in a hurry!".

Hard

There are many elements going on at the same time, polyphonically.  But there is something else that gives the music momentum.  What is it, and how does it interact with the combination of different sounds?