system
Given the following post, respond as if you're commenting on Reddit:
human
Why are train tracks laid on crushed stones/pebbles?

Wouldn't the heavy train vibration move them all around and make the tracks unstable?
gpt
In general, the gravel is laid in order to give the tracks a solid foundation. It also serves to help disperse water and to keep the train above sitting water, as well. 

One of the reasons that trains can be used effectively in certain regions is because the tracks are built using a simple but stable construction. Its very difficult to build roads in certain places due to the ground thawing in the spring and the road cracking or breaking apart because of the unstable foundation. This is why its difficult to build roads in parts of Russia and Alaska. 

The trains have an advantage, because the gravel is very stable, and much less is required to produce a fairly thin track. While roads are very wide and would require several times the gravel to be stable and to avoid cracking unacceptably. 

But to answer your question directly, train tracks actually a built with a machine that vibrates the ground and compacts the gravel. This helps guarantee that the tracks will not erode. 

The rubble is also inspected and any weak areas are fixed fairly easily by an automated machine. This isnt really possible with roads, which require a substantially larger amount of effort to fix defects with the foundation. Since the road itself needs to be torn up, but train tracks can be fixed without even lifting the tracks.