Why is there gravel along the railroad tracks?

Here you go
Track ballast forms the trackbed upon which railroad ties (sleepers) are laid. It is packed between, below, and around the ties. It is used to bear the load from the railroad ties, to facilitate drainage of water, and also to keep down vegetation that might interfere with the track structure. Ballast also holds the track in place as the trains roll over it. A variety of materials have been used as track ballast, including crushed stone, washed gravel, bank run (unwashed) gravel, torpedo gravel (a mixture of coarse sand and small gravel), slag, chats, coal cinders, sand, and burnt clay. The term “ballast” comes from a nautical term for the stones used to stabilize a ship.

It’s called ballast. It’s to fill in the voids that can be created by the train running over the rails and flexing the ties, and what @Forever_Free said. That’s why the ballast fills the voids between the rails to the top of the ties. The same is done to mine car tracks.

Primarily for water drainage.

And, to add to what the others said, anything solid has the potential of experiencing extreme thermal disruption by swelling and shrinking, and thus breaking and throwing the tracks out of line.

When the train moves one the medal rails, they can spark which can cause fires and that is a hazard. It is even more of one when everything is dry so the gravel prevents the sparks from flying into the plants and causing a fire.

I studied about trains.

@Forever_Free Those ‘nautical stones’ were used to cobblestone the streets of lower
Manhattan.

An additional function of ballast is in retarding erosion of the ground supporting the ties and thus the rails. Weather induced events like flash floods and spring melt runoffs play hell with the naked ground.

I have been waiting forever to answer this question.