In the nineties I lived right next to the central Copenhagen train tracks, a bunch of tracks side by side, very busy. (Funny enough one stopped hearing them after a couple weeks, maybe because the sound was not actually unpleasant. Lucky because while outside the city they'd built expensive noise barriers, in the middle of the city, no such thing.)
One night after midnight, a special kind of train passed on the tracks, very slowly. It was making a SPECTACULARLY LOUD noise. Insane. I rushed down there and looked. I was just a few meters from the train, separated only by a fence and a line of small trees.
It turned out it was replacing the rocks between and under the actual tracks!
This was happening with lightning speed; I have no idea what kind of device could suck up fist-sized rocks maybe a dozen per second, but it did, and replaced them with new ones as fast, I don't know why. You can maybe imagine what kind of noise this made.
I was fascinated and exhilarated. This experience was like a huge art happening: the slowly moving, dark train in the middle of the night, with lights on it, the supernatural noise, and the rocks moving up and down like in the elevators of hell, it was amazing.
Aha: the crushed rocks (the ballast) are not always replaced, often they are cleaned to avoid vegetation growth.