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The crushed stones that line railroad tracks are collectively called track ballast. More specifically, the track ballast constitutes the trackbed upon which sleepers or railroad ties are laid. As you may have seen, these stones are packed below, between and around the railroad ties. The thickness of the track ballast usually ranges between 25-30 cm, and varies depending on certain conditions pertaining to the geographical location of the railway track. Track ballast usually consists of crushed rocks or stone, but in some cases, less suitable alternatives, like burnt clay, are also used. However, its important to understand that you cant just throw a bunch of rocks on a railroad and be done with it. As mentioned, the stones in the track ballast have a characteristic shape, size and texture. Their surface is not smooth and their edges are jagged, and for good reason. These rocks interlock with each other and therefore stay in place. If you replace them with circular or smooth rocks, they would slide over each other and compromise the strength of the track ballast. As you might imagine, this can have catastrophic results. Railway tracks are made of steel and other metallic parts that are prone to corrosion and rusting, as they are constantly exposed to the forces of nature, including extreme heat from the sun, rain, vegetation and general wear and tear. The primary reason behind the existence of track ballast is to protect the railway track.