Compound Interest: Why can leaves cause train delays? The chemistry of leaves on the line

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Why can leaves cause rail delays_ — Click to enlarge

Autumn is here, and for most of us, it’s a time of beauty as the leaves cascade through an array of hues before pirouetting down from the trees. If you have to travel by train, however, you might tire of ‘leaves on the line’ being the supposed cause of train delays. It turns out to be more than just a flimsy excuse – and particular chemical reactions are partly to blame.

We’ve previously looked at the chemical cause of the colours of autumn leaves. By the time they make their descent from trees to the ground, most of these colours have passed. What remains is a brown husk, mainly made up of cellulose. Cellulose is the biological polymer that is the main component of plant cell walls.

Once leaves have fallen from trees, they simply decompose over time. Their presence isn’t usually a problem until it comes to the train network. When leaves fall on train lines, they can reduce the grip between the train wheels and the track. This, in turn, can lead to longer braking distances for trains. By disrupting the contact between the train wheels and the track, the leaves also prevent signalling equipment detecting trains. This can then cause train delays.

What makes leaves affect train tracks in this way? Scientists have a few suggestions, and it’s likely that they all contribute to the problem to some extent.

The first explanation is the most straightforward: leaves build up on the track and interfere with the contact between it and train wheels. This is exacerbated in wet conditions, making rails even slipper.

The second possibility is that a chemical reaction can occur between chemical components in the leaves and the iron in the steel rail tracks. This produces iron oxides, which reduce the grip between train wheels and the track.

The final suggestion is that another chemical reaction between the leaves and the iron takes place. Pectin, one of the chemical components of the leaves, can form a pectate gel with iron ions. This gel forms a lubricating paste with the cellulose fibres in the leaves, reducing friction.

Far from being a dubious excuse for late-running trains, ‘leaves on the line’ are serious business – and have serious impacts on train companies. In the UK in 2015, around £50 million was spent clearing leaves from railway lines. Despite this, there were still an estimated 5,800 hours of delays caused by the issue. Unsurprisingly, train companies have come up with a few ways to tackle the problem.

Dedicated cleaning trains blast the leaf residue from the tracks using high-pressure water jets. This can be followed by adhesion modifiers, applied to the tracks to increase grip. ‘Sandite’, a mixture of sand, aluminium powder, and adhesive, is commonly used. There are even signs on the rail network to show where sandite is applied.

In some countries, special trains with lasers fitted to them remove the rail contamination caused by leaves. This process is known as ‘laser ablation’, and has been used successfully in the Netherlands. In the UK, it was trialled in the early 2000s, but the vibrations of the moving trains it was fitted to made it hard to keep the laser focused on the rails. It was eventually passed over in favour of the current water jet and sandite method.

As the demand for rail services continues to increase, leaves on the line are likely to continue to pose a problem for rail operators. As a preventative measure, in the UK Network Rail routinely fells 1,000 trees a week. The company, which manages the 20,000 miles of railway in the UK, has even evaluated how much it would cost to clear all leaf-fall species from railway banks countrywide: an eye-watering £800 million. Based on this, it seems likely that leaves on the line will be eliciting sighs of derision from commuters for many years to come.