Why railway lines has gravels?

Answer 1

From the many combinations that have been tried to support railroad ties, track ballast happens to be the most usable and effective. Its role is to maintain the ties that support the tracks in their position and distribute the load from the wheels on a larger area:

Vertical position: when a train roll by, the huge weight of the metallic mass must not deform vertically the tracks. The gravels because they have holes in the ballast structure can adapt to allow some temporary deformation, distributing the wheels load onto several ties, and then push back the ties into their initial position.

Horizontal position: the ballast holds the ties in position, but adapt to some deformation when the tracks expand in the heat, or retract in the cold. The rock is piled up at the ties level so that it provides a shoulder to the ties.

Aside from its positioning role, the ballast needs to drain rain water, which it does very well being agglomerate. It must evacuate somehow the waste created naturally or by the passage of the trains (dust, fragments of metal...), and avoid grass to grow.

The size and shape of the rock are important. Rocks are crushed and auto-block when they are submitted to pressure.

On may think track beds are just a matter of pilling up some stones, this is not the case, actually this is a full engineering area. Trying to simply pillup stones would just not work. The construction would be destroyed after a few trains have rolled.