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Breakthrough in the fight against “track slip”

Published Nov 01, 2010

Track slip caused by leaves on the track costs SEK 100 million annually, according to a study by the National Rail Administration. A solution to the problem, which according KTH researchers is most prevalent in the Stockholm area around late October/November, is however close to hand. The origin of this slipperiness caused by leaves falling on the tracks is unclear, and intensive research is now being conducted on how it can be most easily detected and best eradicated.

Ulf Olofsson
Ulf Olofsson, Professor and Head of the Department of Machine Design at KTH

Track slip costs society large sums of money each year, both in terms of delays and the material costs that arise due to the damage that takes place on train wheels and rails.

KTH researchers together with SL and the Swedish Transport Administration have studied track slip at Brommaplan and Skogskyrkogården, two areas that have a considerable amount of deciduous trees and that are vulnerable in the Stockholm subway network, and have found out how the problem arises.

“When train wheels crush the leaves, sulphur, phosphorus, calcium and carbon are released, and they become etched into the rails. These are the types of substances that oil manufacturers mix with engine oil for lubrication purposes. These are the pollutants that make the track extremely slippery. So we know why it’s slippery, and how hard this layer of slipperiness is, and its mechanical properties. We also know how much of the rails that need to be washed or treated in other ways,” says Ulf Olofsson, Professor and Head of the Department of Machine Design at KTH.

He goes on to say that it is the combination of low temperature (5 to 10 degrees Celsius), humidity and leaves that cause the problem. And the slipperiness is formed quickly.

“It is sufficient that a leaf is run over 20 to 30 times in order for the slipperiness to arise. It is a very fast process,” says Ulf Olofsson.

To date, one method of tackling the slipperiness has been to run trains much more slowly than usual, which is a poor permanent solution from an economic point of view. It is also important to know where it is slippery, but this is something that is not always easy to see with the naked eye.

“We are currently working with a warning device which warns of slipperiness just like the devices that are fitted to some cars. With the help of optics we will be able to determine where the rails are slippery, where the trained eye cannot keep up,” says Ulf Olofsson.

But the main thing that is happening purely from a research aspect is to determine the most effective and most environmentally friendly way to quickly get rid of the polluting substances and the subsequent slipperiness. The KTH researchers are working here on several parallel lines of investigation.

“One way is to apply sand to the tracks, but this leads to considerable wear and tear on the wheels and the rails. So you have to find the optimal grain size. It is also possible to sand with ceramic compounds. Another way is to polish or grind away the polluting substances which for example can be done with a pressurised cleaner that only uses water or a naturally acidic medium such as citric acid. It is also important to look after the wheels, for example by brake-blocking the wheels to roughen the surface,” says Ulf Olofsson.

For more information, contact Ulf Olofsson at 08-790 63 04 or

Peter Larsson