I hate to burst everyone's metaphorical and symbolic bubbles, but the train sound - or as SC points out the 'el'- had more practical influence on the scene.

Elevated trains in NY are extensions of the subway system. They run constantly and in both directions. They are very germaine to the NY scene in the outer boroughs of the Bronx, Queens and Brooklyn. So to have the train noise in the background is common.

FFC used this commonplace environment to enhance and build the drama of the scene. If you notice Michael fires the gun when the train noise is loudest. From a practical aspect, he waited for that moment to coverup the sound of the gun being heard outside. It also serves the director by adding to the tension and intensity of the scene. The sound of the train crescendos upward, then blam, blam, blam.....silence....then a dramatic infusion of music.

You could say the train noise has some symbolism of what's going on in Michael's head, but I prefer the dramatic effect of tying a commonplace NY element into the drama of the scene.