Ballast: what is it and why do we clean and replace it every night?

High Output teams are out each night replacing up to a mile (1.6km) of ballast.

All about our High Output teams

What is ballast?

Ballast is the name for the ‘stones’ beneath the track. This aggregate forms the trackbed and supports the track. It also helps with drainage, so rain water can drain away rather than pooling, and with preventing vegetation growth, which could destabilise the track and be a hazard for anyone working on the railway.

The ballast is packed up to, between and around the sleepers, with a ‘shoulder’ of ballast piled up at either end to resist side-to-side movement of the track.

Why does it need replacing?

Pieces of ballast are an irregular shape so they can lock together and grip. Over time, they wear down and become rounded. They then lock together less easily, reducing the ballast’s effectiveness.

As the ballast pieces grind together over time through normal wear and tear, it creates fine pieces of granite, like sand. When these ‘fines’ combine with water in the ballast, they stick together: this makes drainage more difficult and the ballast less flexible, therefore less able to restrain the track as trains move over it. 

Ballast cleaning removes this worn ballast, screens it and replaces the ‘dirty’ worn ballast with new angular ballast.


Tampers and stoneblowers also work with ballast, restoring the line and level of the track after renewal work, while ballast regulators redistribute ballast to the right areas of the trackbed to give the track lateral stability.

High Output

Network Rail has invested £300m in High Output in the last 12 years – helping to make our High Output fleet the third largest in the world. We maintain and operate our High Output fleet – maintenance is not outsourced.