system
Given the following post, respond as if you're commenting on Reddit:
human
Why are train rails usually built on gravel?

Is there any particular reason for this or is it just cheaper than asphalt/concrete/whatever?
gpt
The material that forms the trackbed upon which railroad ties are laid is known as **track ballast.** It is packed between, below, and around the ties. It bears the load from the railroad ties, facilitate drainage of water, and discourages vegetation growth that might interfere with the track structure.  It is typically made of **crushed stone**.  

Track ballast typically rests on a layer of smaller crushed stones: the sub-ballast. The sub-ballast layer gives a solid support for the top ballast, and reduces the seepage of water from the underlying ground.

The shape of the ballast is also important. Stones must be **irregularly cut**, with sharp edges, so that they properly **interlock** and grip the ties in order to fully secure them against movement. 

In order to let the stones fully settle and interlock, speed limits are often lowered on sections of track for a period of time after new ballast has been laid.