Here is China Standard Heavy Steel Rail Manufacturer talking about Why Specially Those Stones.
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It’s not like the construction crews put just any stone they find around the tracks. That wouldn’t do the trick. For instance, if you put smooth, round pebbles in the ballast, then they might roll or slide over each other when a train passes over the tracks; therefore, they would fail at their main job – providing solidarity to the tracks. Given that fact, you need stones of a specific type that won’t move around too much, except by kids chucking stones near the tracks, of course!
In order to guarantee that the stones stay in place, they use sharp and edged stones in the ballast.