Why train tracks are laid on top of stones?

A sight like this image above is pretty common. But have you ever wondered why there are stones in between and underneath those rail tracks?

Trains can be extremely heavy, this weight is focused upon the fairly small area of their wheels. The stones are actually called track ballast and help to spread the huge force from the train's wheels out over a larger area of ground. Without this ballast the ground underneath might sink unevenly. The sleepers (cross ties) of the tracks are not directly attached to the ballast which allows the track to have a little movement (e.g. as the track expands and contracts due to changes in temperature).

Stones are a good choice for this role because they are relatively cheap when compared to alternate materials, they can resist compressive loads well and require very low maintenance.

Also of note: Ballast is itself built upon a foundation of earth (the subgrade) that helps to raise the track and further distribute load.

Other functions: Stops plants growing around the tracks, allows water to drain away. Thus it is essential in maintaining the stability of the ground.

Are rocks a significant derailment hazard? Not really -- trains are massive and move quickly -- this enormous momentum means they smash right through most anything in their path. Rocks are typically turned to dust by the wheels of the train or thrown out of the way by the tremendous pressure of the wheels against the track.