Seasonal chemistry

The daily commute for rail passengers can be an uncomfortable experience at the best of times in the UK, but in the autumn, one excuse is often heard from the train companies explaining delays - leaves on the line. It sounds such a weak excuse, how could a few fallen leaves delay the vast bulk of a train? Well, researchers Gordana Vasic and Francis Franklin of the University of Newcastle upon Tyne, UK, working with colleagues in Australia and Serbia, explain the issue with chemistry and a little meteorology. A thin, greasy film of dead leaves can stick to rails causing low adhesion between wheels and track, they say, which not only leads to safety problems with the possibility of collisions but also causes much more wear and tear on the rolling stock. They have carried out tests that demonstrate that friction falls dramatically regardless of the presence of leaves, but on a typical morning during Keats' "season of mists and mellow fruitfulness" dampness combines with nocturnal iron oxidation (rust) and a greasy coating of wet leaves to produce a surface so slippery that no adhesion between train and track is possible.