Why are train tracks often surrounded by large stones?
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If you’ve ever laid something that needs to be perfectly flat like a flagstone or brick patio, planter boxes, etc you’ll see how difficult it is to smooth out the ground effectively since it’s made out of a bunch of different materials with different amounts of compression strength and texture. It’s often mixed with organic material which will decompose and recede. You lay your flagstone and see that one is too high—then what? Lift the whole thing up and re-set it? It would be endless.
Applying a layer sand or gravel with a consistent particle size makes it MUCH easier to rake flat. It’s also more workable because it’s not compacted.
Gravel is great because it resists compaction (the stones sit on each other and don’t change shape even under immense weight) and behaves the same when wet or dry, but if you need to rake it out and lower a raised area or fill in a depressed area, it’s very easy easy to do so. It resists downward force but allows for raking.
When it comes to buildings, foundations will often be made of concrete. The top side is smooth but the bottom is not perfectly smooth and conforms to the underlying material before it hardens. It’s basically the same concept but more permanent.
But concrete is more expensive—it contains gravel, plus sand, plus cement which is made of certain types of fired rock that harden; it takes a lot of energy to mine, heat and cook the cement, and it’s not really necessary in this case so loose gravel will work.
In urban areas, commuter rail or light rail tracks near stations will be set on concrete because it’s more walkable and visually attractive. Just not worth the cost when you’re going out through the rural areas, especially since you’d also need to work our drainage.
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