Why do railway tracks have stones underneath?

Have you ever wondered about the beds of rough, sharp-edged stones found underneath and next to railway tracks? These stones are known as “track ballast”, and they form the foundation upon which the “sleepers” – that’s the large, heavy beams supporting the rails – are laid. The track ballast is first spread out on the bare ground, which raises the level of the track. Once the ballast is laid, the sleepers are positioned equidistant to each other. Additional stones are then packed between the sleepers and around the sides so the tracks can be laid on top.

Bring on the ballast!

But these track ballast beds are not only there to form a base and level out slopes more easily and effectively – they are also used because of the movement of the rails themselves. Yes, you read that right, we’re not talking about fixed objects but about railway line parts that actually move. Contraction and expansion, ground movements and vibrations are all potential problems that can lead to defects in the track.

The ballast stones are always shifting so as to cushion any shocks and vibrations caused by the trains passing over them. The ballast also helps to balance out the temperature differences over the course of the year. Railway tracks are made from steel, which expands when the tracks are exposed to the heat of the sun for weeks on end, and contracts when the temperature drops.

Gaps in the track and “track joints”

Previously, small gaps known as “track joints” would be left between the rails to allow the steel room to expand in the heat. This created the distinctive “clickety-clack, clickety-clack” soundtrack to rail journeys. Despite being considered charming, however, this construction had a significant disadvantage – it subjected the train wheels to major wear and tear. Today, we avoid this shortcoming by welding successive rails together and designing the attachment to the railway sleepers in a different way. This likewise allows the contractions and movements to dissipate thanks to the stony ground.
In addition, the gravel also ensures that rain can drain off easily. This is especially critical in the winter to prevent the railway infrastructure from being damaged by freezing water. 

Further advantages of track ballast

The ballast doesn’t just help the track stay in place; it also plays an important role in keeping any vegetation growing nearby in check. The stones inhibit the growth of roots that might grow under the track and cause misalignment. Finally, having a high number of stones helps dampen noise levels when a train thunders down the tracks. It is hard to imagine just how loud trains would be if there were no ballast bed.

The type of rock is a key factor

Not all rocks are created equal – and neither is all ballast. The rocks used to produce the track bed have to fulfill several different requirements: They should be angular and impact-resistant, as well as free of chemical substances that would change their properties under environmental influences. For example, calcareous rocks are unsuitable for this purpose since water makes them more porous over time. Hard rocks such as granite and basalt are best suited to these requirements, as they demonstrate greater resistance to pressure and abrasion. This means they weather more slowly, too.

What distinguishes the stones used for track ballast is that they are extremely rough and have sharp edges. Using smooth stones like the ones found in a riverbed or in the sea would risk the stones sliding apart from each other as a train passes over, destabilizing the ground. The stones’ sharp edges are useful, as they interlock and can therefore easily absorb shocks and vibrations. 

In addition to their material properties and angularity, the “grain size” of the rocks also plays an important role. Many rail operators have specially developed guidelines and specifications regarding the appropriate grain size.

Success one stone at a time

Track ballast is not a new concept – it has been used for hundreds of years and has stood the test of time. Moreover, the way tracks are laid has hardly changed during this period.
Recently, large sections of high-speed lines, e.g., on bridges and in tunnels, are often concreted, without using any gravel. This is necessary on account of the much higher speeds which can cause suction effects and a risk of flying stones. These routes have to be checked more frequently, however. Nevertheless, ballast beds are here to stay for the foreseeable future, as the advantages of using this inconspicuous and practical set-up clearly outweigh the disadvantages.