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Trains heading towards Bradford on Avon's tiny two-platform station, nine miles outside the historical town of Bath, pass through an avenue of coast redwood trees. The sequoias have been in the area since around the 1830s, when the railway tracks were first laid.
The rows of trees are the tallest lining Britain's 20,000 miles of railway track but they are only a tiny portion of more than ten million trees within 60 metres of the country's railways.
In 2017, Network Rail estimated that 20.1 per cent of the trees next to train tracks in England were from the ash species, 18.1 per cent were oak, sycamores made up 16.5 per cent, while less common Lime trees and Horse chestnuts make up fewer than one per cent each.
In a large number of cases, the trees were there before any trains ever passed through. "The age of the network means that the railway lines weren't built with concrete viaducts, they're all around natural woodland," explains Liam Henderson, who created passenger experience organisation Transporting Cities.
The amount of trees lining the UK's railways causes problems. Leaves fall on tracks and falling trees can block lines or damage electrification cables – all of which leads to delays. As a result, Network Rail, which is responsible for maintaining tramlines around the country, removes the trees it considers may be a problem. This week The Guardian reported an internal Network Rail proposal that could see more than one million trees being targeted for removal.
Network Rail says there isn't a new programme of tree felling and that it hasn't changed its policies since 2004. In response to the reports, government rail minister Jo Johnson announced that all non-emergency tree felling should be suspended during the current bird nesting season while a review is completed. The suspension was welcomed by the Royal Society for the Protection of Birds and groups concerned about wildlife damage that could be caused by the removal of trees.
"I have some sympathy for Network Rail because they're stuck in a balance," Henderson says. "They're being told by the government to reduce delays and costs. One of the delays and costs is the variable of leaves falling and blocking their lines every autumn."
Each time a train is delayed because of a problem with train tracks, which are the responsibility of Network Rail, the organisation is fined. During 2016 the public sector company was fined more than £300 million for delays caused by maintenance and it says problematic trees result in hundreds of millions being lost each year.
When it comes to reducing delays, removing trees can help with problems on the lines. And one of the biggest issues is leaves. "When the leaves have fallen off the tree and land on the rail, they're crushed under the high pressure of the wheels rolling over them," explains Roger Lewis, a professor of mechanical engineering at the University of Sheffield who has researched how train wheels interact with tracks.
"There's a little bit of slip that occurs on the leaves, that creates a high temperature on the contact and that basically causes a chemical reaction to occur that creates a black leaf layer," he says. This layer is a smooth substance that reduces grip on the tracks. The lack of friction can result in trains struggling to slow down and force drivers to accelerate more slowly to stop wheel spin. "When we say low friction we're talking lower than you would get in an oil-lubricated contact in an engine component. It's very low," Henderson explains. He adds that leaves from ash, oak, sycamore and silver birch are "worse than others" when it comes problems on tracks.
Network Rail's tree census, which shows how many trees surround train tracks
Both Henderson and Lewis say disruption caused by leaves isn't unique to the UK. Where there are trains and trees, havoc invariably ensues. So, what can be done about leaves on the lines?
Away from removing trees and foliage, there are more technical solutions available. Lewis says a traction gel could be used by Network Rail. "It's sand particles in a gel that they can put onto the track to try and deal with low adhesion hotspots," he explains. "Most trains also have sanding systems, so when they're braking or in traction then sand is applied into the rail-wheel interface." The sand-based system has been around for decades though, and Lewis argues newer options are needed. His Sheffield-based team is now working on ways to use water to reduce friction.
In 2014, train-mounted lasers were trialled to vaporise the black layer left on the track as a train passed over them. Henderson says one train station in Utrecht, the Netherlands, is flanked by trees that drop their leaves every few months, rather than all at once.
Elsewhere, academics from the University of Birmingham have also developed Internet of Things sensors that can detect moisture on railway lines. Knowing where there's a greater chance of leaf residue building up would allow trains with the technology to clean up more efficiently.
There may also be a more natural solution. Sara Lom, chief executive of the Tree Council, told The Guardian that it is working with rail bodies to help manage vegetation. “Alternatives to removal could be coppicing or pollarding or hedging".
Network Rail itself has been working to improve what it knows about trees around the UK's 20,000 miles of track. It has an adhesion protection tool it uses to monitor areas and in April 2017 revealed it had created a vast tree map. Using LiDAR technology it created heat maps of all the trees surrounding train lines.
This allows it to monitor which trees may fall on lines and where it would need to clear trees to complete maintenance works. "The clustering data shows how many trees exist and you can drive right down to individual trees to uncover a range of different details, from height and location, to tree width," Paul Meads, the head of line-side safety said in a statement last year.
But there's one final catch: trees near train lines may be needed for them to run. "Trees are actually really important to them because their roots hold all the earth together around the track," Lewis says. "If they remove too many they have more problems with landslides. The trees are quite important to the railway network."
This article first appeared on www.wired.co.uk
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