Vacation on the line is a notorious headache for both passengers and trains, which causes costly delays. Now scientists say they have not chosen why fallen leaves make the rails so slippery.
When the leaves are pressed against the tracks, they form a black layer that dramatically reduces the friction between the train wheels and the rails – a situation described by Network Rail as the “black ice of the railway”;. But the composition of this slippery layer was something strange.
Now researchers say they have discovered the main ingredient: large molecules found in plants are called tannins.
“These are the chemicals that make wine dry, and they’re present in tea,” said Dr. Michael Watson of the University of Sheffield, co-author of the new study.
The team claims that the study suggests that tannin leaves are transferred to iron, which has been dissolved from the rails by leaf acids, forming a layer of black material that reduces friction between steel surfaces.
This substance, Watson added, is very similar to ink iron with ink – a substance used for centuries for everything from sacred texts to drawings by Leonardo da Vinci.
Writing in the Proceedings of the Royal Society, Watson and colleagues report how they drew their conclusions by pouring water with sycamore leaves to obtain an acidic brown extract.
When a drop of ferric chloride is added to water in this leaf extract, a black iron-containing material is formed, which is iron dissolved from the rails. This was not the case when tannin-free leaf extract was used.
The researchers then placed tannin-containing leaf extract between pairs of steel surfaces, finding that the black material formed significantly reduced friction between them – compared to situations where only water or tannin-free leaf extract was used.
“The leaf extract itself can cause this extremely low friction,” Watson said.
The team did not compare its black material with material removed from the railway tracks and could not rule out other factors contributing to the slippery conditions.
However, Watson said the results open up new opportunities to reduce the impact of fallen leaves.
“Hopefully, this will lead to some chemical treatments that may stop [the slippery material] from the formation, he told the Guardian.
Watson added that the next step is to study the tannin content of different types of trees, adding that it can help inform Network Network tree felling operations.
“[We want to explore] which of them will have the greatest impact if they are destroyed, and then we will save the rest so that we do not lose this area, “he said.
Zilly Lee, a professor of rail systems and monitoring at Delft University of Technology who did not participate in the study, welcomed the study.
“This is an interesting conclusion, and it should be further investigated in laboratory and railway environments closer to the real phenomenon of slippery rail transport,” he said.