What is the purpose of the large numerous stones all along on either side of a railway track, especially seen in india

The wood or concrete beams beneath the rails are called ties or cross ties. If you are referring to the stones in and around the ties, the stones are called ballast. There are enough air spaces around the individual pieces of ballast to allow rain and/or snow melt to drain away without eroding the ballast away.

I suppose there might be some places where larger stones might be placed next to the tracks as navigational markers where locomotive operators need to sound the horn in order to warn people crossing the tracks, apply brakes or perform other actions related to moving freight or passengers.