Over time, ballast becomes worn, and loses its angularity, becoming rounded. ... This hinders both track drainage and the flexibility of the ballast to constrain the track as it moves under traffic. Ballast cleaning removes this worn ballast, screens it and replaces the "dirty" worn ballast with fresh ballast.

Why do they clean ballast?

As pointed out by a maintenance officer of one of the country's leading railroads, the cleaning of stone ballast improves the drainage by removing the dirt and opening up the voids between the stones.

How do train tracks stay clean?

Ballast is what they call the bed of gravel or other aggregate that sits underneath train tracks, holding everything in place and absorbing the energy of rail traffic. Over time, through, impurities in the ballast can impair its effectiveness.