For many years, British commuters have found themselves caught between bemused laughter and rage when told their train isn’t running because of ‘leaves on the line’.
Each autumn, train drivers suddenly discovered that their train failed to grind to a halt when they applied the brakes and instead started skidded along the tracks. Consequently, all the trains on that line had to go much slower if they were to remain safe and every one of them ran late – if it wasn’t cancelled altogether. It has proved such consistently inconsistent problem, affecting millions of travellers and tonnes of freight, that Network Rail threw some money at researchers at the University of Sheffield and implored them to get to the bottom of the issue.
The only clue was a simple one, the mysterious slipperiness only kicked in after rain. The research team recreated the conditions as best they could, adding the two key ingredients – leaves and water – to a pot to see what would happen. Sycamore leaves have proved to be amongst the worst transgressors and so were used in the research. Next, they added some iron chloride to this soup, to simulate the iron in the tracks, and then they repeatedly tested the slipperiness of their unctuous gloop. Their findings, published by the Royal Society, concluded that tannins were to blame.
Tannins are chemicals present in many plants and their astringent quality helps deter predators. (The name ‘tannin’ comes from the ancient practice of using the chemicals found in oak bark to treat animal skins to create leather, a process known as ‘tanning’.) When the tannins were removed from the slimy leaf soup, the mysterious super-slipperiness disappeared.
Having identified the culprit chemical, new work is underway to test equipment to remove tannins from the railway lines. The lead researcher at Sheffield, Roger Lewis, Professor of Mechanical Engineering, is hopeful that a new dry ice cleaning system will ‘help avoid dangerous sliding events’. Rob Cummings, Seasonal Improvement Manager at Northern Rail, describes it as ‘the next step in finding a solution to tricky autumn conditions’. We all wish them well, I’m sure, but it won’t protect us when on foot. I think the best advice is to be wary of any leaves that appear to be rotting on hard surfaces and give any on metal a wide berth. I view all wet fallen sycamore leaves with suspicion at this time of year, wherever they are. Leaves, algae and more besides, make this a season of slipperiness; take care, but do not be deterred.
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