1) 충격을 흡수해 땅이 흔들리는 걸 막아준다.
2) 비가 올 때, 철로가 잠기지 않다록 한다.
3) 잡초가 자라는 걸 막아준다
https://en.wikipedia.org/wiki/Track_ballast
Track ballast - Wikipedia
Good quality track ballast is made of crushed stone. The sharp edges help the particles interlock with each other. Track ballast (close up) between railway sleepers and under railway track Track ballast forms the trackbed upon which railroad ties (sleepers
en.wikipedia.org
https://www.youtube.com/watch?v=xKPxvTRpYHw
