Patthar k bade gitti tukde jo ki rail patri k neechey sleepers know aas paas aur rail track k dono sides mein aap ne jarur dekhe hongey . Ussi Ko ballast kehte hain . Metro rail par aur even high speed gaadiyon me foreign countries mein Without ballast tracks bante hain ab . Jis me almost nil or bahut kam ballast prayog ho raha hai aaj kal . Thats called ballastless . I hope that explains the point usefully & I think you will agree . Thanx