Blvn Lakshman

Why railway tracks fill with lot of stones?

Its provide the elastic cushion to the sleeper, when the moving load occurs.
Here the list contains the function of the ballast:
(1) It provides a hard and level bed for the sleepers.
(2) It holds the sleepers in proper position during the passage of moving trains.
(3) It provides to some extent an elastic bed for the track.
(4) It transmits and distributes the moving load of the trains from the sleepers to the
formation uniformly.
(5) It protects the formation surface from direct exposure to sun, rain and frost.
(6) It provides a proper drainage to the track, keeping the sleepers in dry condition.
(7) It obstracts the growth of vegetations at the track formation.
(8) It provides proper super elevation to the outer rail on curves.
(9) It provides an easy means for correcting the unevenness of the track.
(10) It provides the lateral and longitudinal stability to the track
(11) It protects the sleepers from capillary moisture of formation.
(12) It provides a media for absorption of all impacts caused by rolling stock.

@#-Link-Snipped-# - You are right, In metro, the track is placed over the cement concrete base, which is designed for static & dynamic. And also the concrete base is connected with the piers to transmit the load to the ground. But if you going to laid the track over the sub-grade. The soil has not the same bearing capacity throughout the track. So using the Ballast as the sub-base to transfer the load.