why do they use gravel on railway lines?

Answer 1

There are several reasons. First, the mass of the gravel is sufficient to provide stability to the rails. Next, the gravel is porous. Water drains from the tracks easily. Finally, gravel will both prevent erosion and will hold back growth.

Answer 2

The gravel is an extension of the cross tie
system.
It serves to spread the weight applied to the
rails over a larger area of ground while
providing drainage to keep local 'soft spots`
from developing in the underlying soil.

Answer 3

To hold the soil and retard vegetative growth.