Have you ever wondered why the train tracks rest on cobblestones and stones? Some might think that they are there to prevent weeds from growing, but in reality those without the right maintenance would grow anyway. Let’s see what all the functions of the gravel found on the tracks.
What is railway ballast?
The pile of stones you see around the tracks has a precise name: it is in fact called “ballast”, or “ballast” in English. In general, a ballast is composed of gravel placed on a surface and serves as a support surface for infrastructures such as roads or, indeed, tracks. While in English “ballast” is an abbreviation of “track ballast” and means “ballast for tracks”.
To perform its function at its best, the ballast must be of aminimum heightwhich is usually of 15 centimeters where the lighter trains pass, and that It can reach up to half a meter for high-speed lines or where very heavy vehicles pass. Above this height the benefits are negligible. In high-speed lines, moreover, this layer rests on another substratemade of smaller stones, which serves to better waterproof the ground. In practice, there are two levels. Even the hill shape is not random, but serves to better allow rainwater to flow so that it does not settle on the tracks.
What characteristics should stones have?
The stones used must have some precise characteristicsand depending on their origin they can have a different “expiration date”. What does this mean? It means that the ballast is removed and replaced in its entirety approximately every 20-30 years, a duration that can vary depending on the use rocks of sedimentary or volcanic originmore resistant. This is because the passage of trains produces a slow but continuous grinding of the gravel, which therefore needs to be replaced cyclically.
The stones are chosen for their Frost and friction resistanceand are on average between the 3 to 6 centimeters in diameter. Looking at them more carefully you will then notice that they have a rather square cut: this is because the edges allow the stones to fit together well while still leaving the space needed for the water to flow. Rounder stones would instead end up sliding over each other, dispersing.
What is the purpose of ballast?
The first function of the ballast is to absorb vibrations exerted on the rails. This type of fragmented surface is in fact able to deform depending on the stresses, which in the case of a moving train are truly considerable. In the long run, solid ground would risk fracturing, with the risk of collapse, just as the sleepers would risk detaching. The ballast, on the other hand, allows these vibrations to be absorbed. This also has an indirect benefit on train passengers, who feel a sensation of stability even when travelling at very high speeds.
The mound of stones then helps to distribute the load of the train over the entire strip of ground of the tracks.. Without the ballast, the train would concentrate a high weight on a very small surface, with a high risk of collapse. With stones, this pressure is also distributed better.
Another function of the ballast is to allow the tracks to adapt more easily to the altimetric configurationthat is, curves and differences in level. The stones allow you to better shape the support surface, thus making the transition from a straight line to a curve more harmonious or a change in slope less sudden.
The ballast also serves to counteract the effects of heat in summer. In addition to absorbing heat, taking it away from the tracks, the ballast stones also allow the tracks to shrink and expand with a certain safety margin, which they would not have if they were in direct contact with a solid surface.
Maintenance of the ballast
In short, as you can see, the functions of the gravel are many, and in fact the ballast requires continuous maintenance to ensure that the stones continue to perform their function correctly, or rather their functions. Maintenance is also essential to prevent the ballast from being covered by vegetation or, worse, by mud and dust, thus losing the ability to drain water.
Alternative solutions
However, ballast is not the only possible solution for a railway. It is certainly the most widespread, because it allows the tracks to be adapted to practically any type of surface, but it is not uncommon for tunnels – or on terrains that allow it – to use reinforced concrete platforms, resting on rubber or polyurethane mats. In this case, it is the mat that has the task of absorbing vibrations and distributing the load. A technological solution that is not necessarily always the most suitable.
