The season of mists, mellow fruitfulness, and leaves on the line is almost upon us. But, scientists have discovered that rather than blaming the train companies passengers suffering annual delays should look to the weather forecast.
According to research published in the International Journal of Surface Science and Engineering, it is a dry period followed by drizzly or misty weather that leads to the worst conditions for leaves on the line.
Mechanical engineers Gordana Vasic and Francis Franklin of the University of Newcastle upon Tyne, working with colleagues in Australia and Serbia explain how a thin, greasy film of dead leaves can stick to rails causing low adhesion between wheels and track. This not only leads to safety problems with the possibility of collisions but also causes much more wear and tear on the rolling stock.
Under laboratory conditions, with clean and dry rail steel, the amount of friction between rail and wheel is usually high and means that trains run smoothly. In wet conditions, friction, known in the industry as adhesion force between wheel and rail, drops by half. However, the adhesion force can fall dangerously low on a typical autumn morning, for instance, when dew or dampness on the rails combined with overnight rusting and a coating of wet leaves conspire to produce a slippery surface.
The team explains that leaves on the line is not simply a matter of a few loose leaves falling on to the track and being caught up by passing wheels. Instead, fallen leaves are whipped up by the slipstream of passing trains, crushed under the wheels and in damp conditions form a continuous greasy film, like a black paint, on the rails.
"The film is very difficult to scrape from the surface," the researchers explains, "and freshly fallen leaves, get spread along the length of the track by train wheels so that the problem becomes widespread, not localised to areas with lots of trackside trees and plants."
The main problems that result from this occur when the adhesion force between train and track is not sufficient for traction or braking. Problems with braking lead directly to Signals Passed at Danger (SPADs), trains overrunning station platforms and sometimes even collisions. "Sudden drops in adhesion can cause havoc with timetables, and can lead to accidents electrical insulation by contaminants can cause problems with track circuitry," the researchers explain.
The team points out that mitigating the problems associated with leaves on the line costs UK Railways about £50 million each year of which £25 million is spent on a fleet of high-pressure water-jet rail-cleaning trains, £10-15 million on vegetation management and £5 million on "leaf buster" teams. The remaining £10 million is compensation to train operators to cover wheel repair costs.
Vasic and colleagues have now carried out laboratory studies on leaf-film formation and measured just how slippery leaves on the line can be depending on atmospheric conditions. It turns out that misty conditions are found to be the best for producing low-adhesion leaf films.
They hope that the precise scientific details of their work will help engineers understand more about the physical and chemical nature of leaf film. Such understanding might lead to improved methods for prevention or removal and one day allow rail passengers to enjoy travelling during the season of mists without this reason for delays.
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