Railway tracks are neither concrete nor asphalt; why are stones packed between the ties?

If you've ever waited at a railway crossing or stared out an Amtrak window, you've probably seen it: mile after mile of loose, jagged grey stones sitting under and around the rails. It looks almost like an afterthought, as if nobody got around to paving it properly. But that “unfinished” look is actually one of the most carefully crafted parts of the entire railroad.

But what are those stones doing down there, and why is it so much harder than it looks to keep them from sinking or shifting? A 2021 engineering study in the journal Transportation Geotechnics took a hard look at all that. The study titled ‘Modelling railway ballasted track settlement in vehicle-track interaction analysis,’ by engineers from the University of Southampton and the University of Huddersfield, has found that even track in good working order gradually sinks with every train that passes over it. The amount per pass can be as little as a nanometer, but multiplied across the millions of wheel passes a busy line sees over its lifetime, it adds up to real, measurable settlement that engineers still find difficult to predict precisely.

It's called ballast, and it's not just gravel
Those stones have a proper name: track ballast. And no, it’s not the same stuff in your driveway. According to Science ABC, the rocks used are sharp, angular crushed stone, usually granite, basalt, trap rock, quartzite, or hard limestone, and about 19 to 63 millimeters (roughly three-quarters of an inch to just under two and a half inches) across. The jagged edges matter. Smooth and rounded, like the kind of pebbles you find on a beach, would just roll about under the weight of a train. Angular stones lock into each other instead, a bit like how a dry-stone wall holds together without any cement.

Why not just use concrete, then?
This is the obvious question, and it's a fair one. Concrete and asphalt exist; they're strong, and we use them for basically every road in America. So why hasn't the railway industry just paved over the whole thing?

Turns out, rigid surfaces create their own problems. Any train that goes over it must spread its weight so it does not crush the soil underneath. The Federal Railway Administration’s track-inspection page notes that the ballast and the layers beneath it form the roadbed that carries this load into the ground, and keeping that roadbed intact is a major focus of federal track-safety standards in the United States.

A loose stone bed also flexes under each train, and that small amount of give helps soften the impact instead of sending the full jolt into a rigid surface. There is a rigid form of track, sometimes called slab track, and it is used on some newer high-speed lines around the world. But loose stone is still far more common. Mostly because it is cheaper to build, and much easier to dig up and reshape if something needs fixing, than having to break apart a solid slab.

Drainage is the other half of the story
Water is one of the main threats to a train track. If water pools around the rails, the ground underneath can soften, leading to sinking, shifting, and eventually a rough, unsafe ride. According to Network Rail, which maintains the tracks across Great Britain, ballast is regularly redistributed and compacted with specialized machines precisely so that it can maintain both its drainage function and the exact geometry of the track. The gaps between the angular stones allow rainwater to drain straight through and away from the rails rather than collect on the surface.

The ground never really stops moving
And here’s where the Transportation Geotechnics study comes in: even on a healthy, well-built track, the ballast settles by an almost immeasurably tiny amount with each passing train. The study says settlement per pass on a well-performing track can be on the order of a nanometer. But multiply that by the millions of wheel passes a busy line sees over its lifetime, and it adds up to real, measurable sinking. That is part of the reason that maintenance crews periodically send out machines to lift, realign, and re-compact sections of track, a process called tamping.

The same research also found that once a section of track begins to sag, it tends to worsen faster. Trains hit a dip, bounce harder as they pass over it, and the extra jolt pushes the stones down further, which makes the dip deeper, and so on. It’s a feedback loop. The authors note that engineers are trying to build better mathematical models of settlement, rather than relying on older rules of thumb, to understand it more accurately.

Next time you're waiting at a crossing
It might seem too easy to write those stones off as filler. They aren't. They are doing several jobs at once: spreading out enormous weight, draining off water that would otherwise destroy the track, and flexing just enough to absorb the shock without ever letting the rails rest on anything hard and brittle. This is a design that has been used since the early days of railways, and engineers are still using modern science to model and improve it. So next time your commuter train rattles over a stretch of track, you’ll know what’s quietly doing the heavy lifting underneath you.