system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why are train tracks surrounded by tonnes of loose stones?
gpt
It keeps the rails in the right place.

Absent any kind of substance around the rails, they'd eventually move around due to the extremely heavy trains going over them all day, so you need something hard that keeps the track from sinking or moving side-to-side.  It needs to be something that doesn't expand or contract when the temperature changes, because that would cause the problems you're trying to solve.  It needs to allow good drainage, because otherwise the rails would rust and the ties would rot.  It needs to be inhospitable to plants, otherwise they'll grow on the tracks and eventually eat the railroad ties or their roots would cause issues with alignment.

It turns out that stones are the best solution they've come up with that fits those constraints, and it's relatively cheap besides, which is a nice bonus.