Any railway track has three main segments – the steel track on which the train runs, the wooden or concrete railroad ties placed perpendicular to the tracks, and the crushed stones that form the trackbed upon which the railroad ties are laid. These stones packed between, below, and around the ties are called track ballast.
The primary purpose of these stones is to keep the railroad ties in place. (The ties, in turn, hold the tracks firm and properly spaced.) The ballast also holds the track in place when trains run over it and keep vegetation in check that might interfere with the track structure. Weeds and plants can also make the ground beneath the tracks weaker. Do you know why sharp, crushed stones are used in particular? Smooth, round pebbles might slide over each other when a train passes over the tracks, whereas sharp-edged stones ensure that tracks stay in place when super-heavy trains roll by on them.
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