At a casual glance, they might look like regular pebbles, but if you ride on trains frequently, you may have noticed that regardless of the country in which you're traveling,

the shape and texture of those stones are almost always the same—all across the globe!

So what's the reason behind that? 

Why do such otherwise unremarkable rocks line our planet's railway tracks? 

The crushed stones that line railroad tracks are collectively called track ballast.

More specifically, the track ballast constitutes the trackbed upon which sleepers or railroad ties are laid.

As you may have seen, these stones are packed below, between and around the railroad ties.

The thickness of the track ballast usually ranges between 25-30 cm, and varies depending on certain conditions pertaining to the geographical location of the railway track. 

Track ballast usually consists of crushed rocks or stone, but in some cases, less suitable alternatives, like burnt clay, are also used.

However, it's important to understand that you can't just throw a bunch of rocks on a railroad and be done with it. 

As mentioned, the stones in the track ballast have a characteristic shape, size and texture.

Their surface is not smooth and their edges are jagged, and for good reason.

These rocks interlock with each other and therefore stay in place.  

If you replace them with circular or smooth rocks, they would slide over each other and compromise the strength of the track ballast.

As you might imagine, this can have catastrophic results. 

Railway tracks are made of steel and other metallic parts that are prone to corrosion and rusting, as they are constantly exposed to the forces of nature,

including extreme heat from the sun, rain, vegetation and general wear and tear.

The primary reason behind the existence of track ballast is to protect the railway track.

It does this in a number of ways. 

1. Holds the sleepers in place.

Many trains, including cargo and passenger trains, roll by on railway tracks every single day. 

Therefore, it's not surprising that those tracks must constantly withstand a great deal of stress and pressure. 

Track ballast holds the sleepers in place, so that they don't move longitudinally or laterally when heavy trains roll over them.

Without track ballast, those sleepers might give away under the constant pressure of passing trains.

2. Load distribution.

Trains are immensely heavy and exert massive pressure on the tracks and sleepers. 

Since stones in the track ballast are jagged, they interlock with each other.

This way, the immense load that's exerted on the sleepers and track gets distributed evenly through the track ballast. 

3. Vibration dampening.

Track ballast absorbs the vibrations caused by a moving train.

The cushioning effect provided by the stones in the track ballast adds o the comfort of passengers riding in the train. 

To test this, next time you ride in a train, pay close attention to how you feel when the train passes over a stretch of railway track without track ballast.

4. Drainage.

It's important to ensure that railway tracks don't get flooded by rains or water clogging. 

Track ballast comes in handy in this regard too. 

It prevents water clogging or the accumulation of water on or around the sleepers. 

Without it, water would accumulate on the tracks and cause rapid rusting of their metallic parts. 

5. Prevents vegetation around the tracks.

Although we all enjoy swaths of land filled with green grass, plants and trees, vegetative growth can wreak havoc on the strength of a railway track.

It's extremely important that there is no growth of grass, weeds or plants around the tracks.

The stones in the track ballast greatly contribute to keeping out vegetation that could compromise the structure and strength of the track. 

6. Noise absorption.

A moving train is quite loud, especially to people who are OUTSIDE the train.

The crushed stones in the track ballast offer a larger surface area than a flat plane.

This greatly facilitates noise absorption. When trains move on tracks that don't have track ballast, they are noticeably louder.

7. Minimizes heat expansion.

During the summer season, railway tracks tend to expand due to the heat.

Track ballast helps keep this heat expansion to a safe minimum, ensuring the efficacy and safety of the track. 

As you can see, track ballast not only protects the railway track and its sleepers from the forces of nature and the stress of a moving train, but also adds to the overall comfort of passengers. 

Looking at those stones, you may think that once they're deposited in the tracks, they just lay there forever, without ever being disturbed.

Well, that's not quite true.

If the ballast is covered in too much dirt or mud, it will affect its effectiveness; it can cause debris to accumulate between the stones and negatively impact its draining ability. 

Just like the railway track, track ballast requires regular maintenance and cleaning. 

Sometimes, the stones are replaced with a new set of crushed stones. 

There are also machines called ballast cleaners that specialize in cleaning track ballast, which reduces the amount of manual labor required.

It's amazing how such a seemingly random set of stones constitute the backbone of an industry that affects millions of lives around the world!