What’s the quietest thing you’ve ever heard on the rails?

Last night, just before dawn, I was idling at the yard switch. The air was so still it felt like the world had paused. No wind, no distant engine—just the faintest hiss from a valve slowly bleeding off pressure. That sound, almost imperceptible, lasted maybe ten seconds. Then silence again. It wasn’t a failure. It wasn’t a breakdown. It was just… something breathing. I’ve been thinking: what do we miss when we only listen for the loud? What’s the quiet that holds the weight of everything?