Why do railway tracks have small quarry rocks??

Question

Why are small quarry rocks mostly used to fill between sleepers on a railway track? atleast this is so in India. Why is this??

Answer 1

Because the tracks are on the ground, the ground moves, and when the train moves over the tracks the tracks move, which moves the ground. The rocks keep the tracks in place from shifting too much, but also allows the tracks to move with the ground. If it was cement around the tracks, the tracks would crack the cement every time the train passed by.

Answer 2

That is what is called (at least in the U.S.) "Crushed rock ballast"

You'll find that on rail systems all over the world.  The reasons were well stated by Peter M but I will also add that it helps in drainage of water when it rains or if the area floods.