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Techly Explains: Why are there crushed rocks next to train tracks?

Have you ever looked at a railway line and wondered why they are surrounded by rocks? Was it simply an aesthetic design that never really changed over time, or do they actually have a purpose?

Turns out, they actually do have a purpose, and a rather important purpose at that.

These rocks, known as ballast, are crucial to keeping railway lines secure and in place. To really understand why they are so important, we need to take a look at the railroad track composition as a whole.

Firstly, for rocks to be used as a ballast, they must have sharp, square edges in order to be effective — round, smooth rocks would simply roll and slide away from each other, and that could only end in disaster.

Lying on top of and surrounded by the ballast are the railroad sleepers. These are traditionally made of long, thick pieces of lumber, however more modern rails can use sleepers made of composite plastic, steel or concrete.

The sleepers aren’t actually held in place by any mechanical methods, and are simply held down by the weight of the rails lying on top of them, and prevented from shifting by the sharp edges of the ballast.

The actual tracks are then anchored to the sleepers, instead of being bolted down, to allow the steel beams to expand and contract as they heat and cool over the course of the day.

It’s worth noting that if the railings were bolted into place, they would quickly bend and buckle as the metal shifts with the temperature, but by simply anchoring them to the sleepers, they are able to shift in place without coming away from their foundations.

Train track cross section

This method for laying railroad has remained largely unchanged for over two centuries, in one of history’s best examples of ‘if it ain’t broke, don’t fix it’.

However, while this method can be improved, and ballastless tracks do exist. These are uncommon, however, due to the higher initial expense — despite lower ongoing maintenance costs — and the difficulties in converting already existing tracks.

In order to change a track from traditional foundations to newer, balastless foundations, that whole line would need to be closed off for the duration of the works — which is why railroads in Australia remain almost exclusively lined with ballast.

So the next time you’re on a train, it might be a bit creepy to think that there is nothing actually securing you to the ground, and the rails on which you roll along are held in place simply by a pile of crushed rocks.

But don’t worry too much — it’s been two centuries since it was thought up, and it seems to have worked out pretty well so far.

About the author

Tyler is currently based in Canberra, though he rejects this reality and enjoys immersing himself in games, technology, and comics. You can usually catch him trying to find that last shard/flag/feather.
Twitter: @FinalAlchemist

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