The train moves very fast and when the train moves the tracks vibrate very fast. Due to this vibration, the tracks do not move here and there, therefore, pieces of stone are put around the tracks of the rail. 
They act as a spring on the tracks. When the train passes through the tracks, they are buried downwards and when the train passes, they return to their place.
Another advantage of these stone pieces is that there will not be bushes around the tracks and the tracks can move here and there.