why do they put rocks on the train tracks in some places but noy in others like in tunnels?

Question

i mean whats the reason behind this?

Answer 1

The rocks are called ballast. Ballast helps to stablize the track. The track can shift some from one side or the other with hot temperatures and places where the train brakes are always applied. More importantly, the ballast allows rain water to drain on through, where as dirt retains the water and rots the ties faster. In colder climates, the ground freezes in the winter, and ground with a high water content heaves and moves, just like ice does. The ballast acts as a cusion between the tracks and the frozen moving ground. I live in Minnesota and the ground can move a lot here in the winter. I have seen slow speed railroads in the south and west where no ballast was used. Water isn't an issue there. I don't know about the tunnels, but I suspect no ballast there because there's no water and maybe slower speeds with in.

Answer 2

Indeed, as others have said ballast (i.e., rock or stone) is merely a support base for the railroad track structure. Along with providing support and holding the track structure in place, ballast also acts as a cushioning agent as well as a way to properly drain water quickly away from the rails (long term exposure to water is very damaging to the rails).

Of note is that most tunnels do include at least a little ballast inside or at the very least some means of sub-ballasting support for the track structure (as is found under almost all railroad right-of-ways, a very hard, compact, surface [usually either concrete or pavement, which is often used today] that supports the main infrastructure [i.e., the ballast, rails, ties, etc.]).

Answer 3

They don't put rocks on the train tracks. Railways, like roads need to be built properly and what you call rocks is known as ballast. It provides a foundation and drainage for the tracks. You couldn't just lay track straight onto the ground - just think how uncomfortable the ride would be - and it wouldn't last very long before the train fell off. There is also ballast in some tunnels - even there a foundation is necessary. However, in very long tunnels the track is often laid on to a specially constructed concrete foundation.