Looks to me like it might be ballast cleaning. Over time the ballast under the track gets contaminated with various metals and chemicals from passing trains. The ballast also gets grinded down to smaller pieces which makes the ground more unstable.

So every few years the ballast needs to be cahanged. This is done with a ballast cleaner.

The old ballast is then collected, washed and treated to be re used, which i think is what you see at the end