Tannins are responsible for delays caused by ‘leaves on the line’

Scientists have finally identified the culprit for why leaves on railway lines are such a hazard and nuisance for commuters. 

A groups of chemical compounds found in many leaves, known as tannins, are thought to be responsible. 

In plants, these large molecules are used to soak up disease and stop it spreading, as well as making the plants taste bitter in order to discourage herbivores from eating them. 

But when the leaves fall on to a railway line, these tannins react with charged iron particles from the steel rails to form an extremely low-friction black paste. 

When compressed by a trundling train, the paste becomes viscous and prevents the wheel from touching the rail, creating extreme safety concerns for rail operators.  

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‘Leaves on the line’ is a dreaded term for many commuters and can lead to lengthy delays, and expensive problem for train operators. 

National Rail is working on ways to negate the issue which caused 3,700 hours of delays in 2017. 

The Rail Safety and Standards Board estimates that autumn-related issues cost the railway industry approximately £345 million per year.

Discovering why leaves pose such a problem has taken years of research, due to the complexity of chemicals and metabolic pathways found in plants.

Sycamore leaves are known to create the slippery black substance, which is such an effective lubricant. 

It reduces friction between wheel and rail to around 0.01 per cent – the same as between ice and a skate. 

University of Sheffield academics took the leaves and soaked them overnight to mimic the leaf falling during a sodden autumn evening. This produced a slightly acidic brown liquid. 

When leaves fall on to a railway line, tannins react with charged iron particles from the steel rails to form an extremely low-friction black paste. When compressed by a trundling train, the paste becomes viscous and prevents the wheel from touching the rail (stock)

Researchers then introduced aqueous iron into the solution, to replicate the steel of the rails. 

This successfully created the troublesome black precipitate known for its friction-reducing properties. 

To find out what chemical in the leaves was responsible for the paste, the researchers used a process of elimination. 

Different chemicals were removed but the paste still formed. However, when tannins were removed from the equation, the paste did not form. 

When this so-called iron tannate mixture was placed between two steel surfaces, to mimic a rail and a wheel, it slashed friction to a fraction of what water alone creates.

‘With this chemical reaction found and linked to low friction phenomena, interventions can be designed to target the specific reactions or chemicals,’ the paper concludes.

Currently, National Rail sends out a fleet of 61 specialist trains which wash away the greasy mulch created by wet leaves.   

However, the researchers say this discovery could help inform policies designed to fell problem trees. 

For example, instead of removing all trees in a specific vicinity to railway lines, it may be better to just cut down tannin-laden species and leave the trees low in the pesky chemicals. 

However, another solution may lie in stopping the tannins from the leaves interacting with the ions in the steel at all.  

‘Interventions can be designed to target the specific reactions or chemicals’, the authors said.

One way to do this would be to apply a chemical which is better at binding to the iron ions than tannis, such as chelate. 

The research was published in the journal Proceedings of the Royal Society B.