Understanding Leaf Chemistry and Effects on Friction

The University of Sheffield, alongside industrial partner RSSB, is investigating the formation of "black leaf layers" as well as developing a model to predict the likelihood of leaf induced low adhesion at specific locations.

Rail with low adhesion black film from crushed leaves
Rail with low adhesion black film from crushed leaves

Black leaf layers are formed when leaves of different species are squashed by train wheels trains during rolling and/or sliding passes. Under certain environmental conditions, such as low surface moisture, they are known to reduce adhesion to below critical levels. This not only constitutes a major safety risk but contributes to serious financial costs to the industry as a whole. These costs can be attributed to Autumn performance dip, driver training and seasonal fleet costs including rail cleaning, repairing damage and lineside vegetation management.

Layers formed from different species are being investigated, as are hypotheses for the bonding of the layer to the railhead, and for how the layer reduces adhesion levels. This involves a combination of physical testing and chemical analysis.

Additionally, a model is being developed using data on Wheel Slide Protection (WSP) activation provided by Chiltern Railways Company Limited (CRCL) to form a case study. This study involved the analysis of historical WSP data to identify high, medium and low risk sites, then investigating them using a combination of physical visits, signal diagram analysis and where necessary virtual investigation using Google Maps and Streetview. Locations were assessed using 10 physical parameters, the relative weighting of which were found using a mathematical analysis. The resulting model assesses locations and produces a risk category rating. When applied blind to the second half of locations from the CRCL study, the model was able to predict (or overpredict as a safety precaution) the risk when compared to the historical data.

People involved

Professor Roger Lewis

Tom Butcher


Chiltern Railways Ltd

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