Every year, rail passengers and operators face the problem of ‘leaves on the line’, which cause delays, timetable changes and safety issues. But what exactly are these leaves and how do they turn into a slippery layer that reduces the friction between the train wheels and the rails?
A team of engineers from the Universities of Sheffield and York have revealed the chemical mechanisms that take place when leaves on the line are crushed between the wheels of a train and the railhead, forming a thin film that sticks to the metal surface. This film is composed of certain chemicals, such as polyphenols, including tannins, which are also found in wine and tea. These chemicals react with iron and oxygen to create a durable and low friction layer that is equivalent to the ‘black ice’ of the roads.
The researchers used a combination of experimental and computational methods to analyse the composition and structure of the leaf-derived film. They found that the film contains complex organic compounds, such as quinones and phenolic acids, that are derived from the breakdown of plant cell walls. These compounds have strong adhesive properties and can form bonds with the iron oxide layer on the railhead. The film also has a low shear strength, meaning that it can easily slide under the pressure and heat generated by the train wheels.
The findings, published in the journal Proceedings of the Royal Society A, could help design more effective solutions to the problem of leaves on the line, which costs Network Rail an estimated £350 million per year. The researchers suggest that targeting the phenolic chemicals, such as by using enzymes or cleaning agents that can dissolve them, could be a way to restore the friction and improve the performance and safety of rail transport.
Dr. Joe Lanigan, the lead author of the study from the University of Sheffield’s Department of Mechanical Engineering, said: “The purpose of the research was to find out how tree leaves transform into the low friction black layer that is a problem for railways. We hope that our findings will contribute to the development of new technologies that can tackle this issue more effectively and efficiently.”
The table below summarizes some of the key features of the leaf-derived film and its effects on the railway system:
| Feature | Description | Effect |
|---|---|---|
| Composition | Contains polyphenols, quinones, phenolic acids and other organic compounds | Forms a strong and thin film on the railhead |
| Structure | Has a layered and crystalline structure with bonds between the organic compounds and the iron oxide layer | Increases the durability and adhesion of the film |
| Friction | Has a low shear strength and a low coefficient of friction | Reduces the traction and braking ability of the train wheels |
| Remediation | Can be removed or degraded by targeting the phenolic chemicals with enzymes or cleaning agents | Restores the friction and enhances the operational performance and safety |
