All quiet on the southern front...

I experienced something immensely thought provoking this week. As I have a new job downtown, I've been taking the train to get there. Now, I take the train quite often, and it's always noisy when I do. But something is different in the morning, as more and more people board at each stop, the train gets quieter, and quieter, until finally we're all standing mere inches apart and the car is filled with silence. The only sound you can hear is the occasional page turn of someone reading a novel at the other end of the car. We stand there, drifting forwards and back, side to side, like seaweed driven by the currents, until we all arrive at our destinations. Why is does more people = less noise?