Directly Under the Seat: This Is Why Many Trains Always Have Sand on Board

Many people travel by train or tram every day, but few notice this clever piece of technology at work, and it could be hiding just under your seat.

Despite all the jokes about punctuality issues and frustrations over strikes on the railway, the technology behind trains is fascinating. Most of it, however, remains hidden and unseen by passengers.

One of these technologies is slightly different and can be found underneath certain seats. This is the sand container - a large box where sand is stored. The container has a small window through which you can see large quantities of sand, you may have even seen this before on your travels. 

It may seem odd for trains to have sand on them, however it serves an important purpose: the sand is needed to assist with braking the train, especially in wet conditions. The wheels and tracks are made of metal and easily slide over each other, reducing rolling resistance and saving energy. However, this becomes a problem when stopping.

Under the sand container, there is a spreading device that can be easily seen from the outside of the train. This device allows the sand to trickle onto the track whenever the train's control system detects that an axle is slipping. Alternatively, the train operator can trigger the spreading manually – for instance, when wet leaves are already visible on the tracks.

For this purpose, clean quartz sand is used because it consists of very small and round grains that trickle easily without clumping. Compressed air helps to blow the sand out through the hose end, aimed at the wheels. 

The sand increases friction between wheels and rails, much like sandpaper. This significantly reduces the braking distance of trains and trams. The ground sand is also responsible for the distinctive smell at the platform when a train stops, similar to the scent of cap guns used on New Year's Eve.

Sand spreading devices have been in use for decades. Initially operated manually, these devices did not allow for precise dosing of the material. On modern trains, sand is mixed with a form of gel so that the product is evenly and quickly applied to the tracks. It may seem a fairly simple piece of technology, but it remains one of the most important.