I agree with Don Vercetti. I would only add that the viewer should note that the sound of the train gets louder & faster. This is a metaphor for the fact that once Michael kills Sollozzo & McCluskey, then Michael's life will never be the same again and there's no going back. It represents all of the angst that Michael is feeling. Notice how he can't concentrate on anything Sollozzo is saying just before pulling out the gun.