Eli5: Why are railroads full of rocks?



Eli5: Why are railroads full of rocks?

In: Engineering


Simplified: You need an element to take on the energy from the passing train and distribute it. It needs to be heavy not to just fly away. It needs to be flexible so it doesn’t break, which is concrete or solid materials aren’t a good idea (at least not with high speeds. You can use it when the train is going slow like at crossings or such).

Ballast is a comparably cheap way of leveling ground that can support a lot of weight very cheaply.

Rocks don’t wash or blow away, they allow for thermal cycling, more or less self leveling, easy to repack when they do sag, easy to add new material to.

Compared to say a concrete foundation, they won’t crack, and doesn’t require formwork during construction, and more importantly don’t require extensive ground works before it can be placed.

It is called ballast. The ballast is very stable and does not move whenever a train runs over it. So it will effectively transfer the weight of the train over a much larger area of the ground and preferably all the way into the bedrock. However the ballast is movable whenever there is constant forces. So if the ground moves the ballast will just adapt as the individual rocks will be able to fill any voids and get pushed away from any high spots. Similarly the rails gets longer and shorter depending on temperature and will move on top of the ballast. The ballast also helps with things like drainage and to keep the weeds away.

The term you’re looking for is [“Ballast”](https://www.google.com/amp/s/slate.com/human-interest/2013/09/why-are-there-crushed-stones-alongside-rail-tracks.amp) which is a type of [“gravel”](https://en.m.wikipedia.org/wiki/Gravel).

To answer your question: the rails shift slightly, and expand/contract as the heat and cool throughout the day. Furthermore, the sit unburdened 99% of the time, but occasionally have to support weights of 1,000,000lbs or more. The layer of ballast keeps the track from moving without firmly securing it in place (which would snap the rail from the nontrivial expansion/contraction cycle).

The first link I posted has a solid explanation (which I’m paraphrasing here)

While all of these answers are interesting, the main reason for all the “rocks” or ballast as others have pointed out are for drainage. If they just placed the rails on soil then eventually rain water and mud can effect the tracks in a bad way. Think like, washing out the soil completely from under the rails which could cause a major crash. Drainage is key for railroads.