For railway commuters across England and Wales, December brings to an end the annual ‘Leaf fall timetables’ and the regular announcements of ‘leaves on the line’ causing delays.
Leaf and other contamination such as rust or grease on the railhead in conjunction with low levels of moisture from rain, dew or frost, can result in extreme low adhesion on the railhead. Low adhesion in simplistic terms is the slipperiness of the rail, and means that the ability of trains to brake and accelerate are frequently compromised. For some headline writers this annual event is symptomatic of how the modern railway is struggling to cope with an adhesion problem that occurs each year. After all, how hard can it be to clear falling leaves off a railway track? But we should spare a thought for the science and engineering that is going on behind the scenes and how hard the rail industry is working to understand and overcome the problem through a multi-faceted RSSB research programme.
Every year deciduous trees lose their leaves, and deposit tonnes of leaves on the railway line. The repeated crushing of leaves under the wheels of passing trains, each of which exerts a significant force of over 5 tonnes per square centimetre (over 30 tonnes per square inch), results in a very slippery residue on the rails. This in turn causes wheels to slip when trains are moving off and when stopping in stations or at red safety signals – a serious safety concern. The exact date and how quickly a deciduous tree can lose its leaves varies significantly each year. Timing and intensity of falls also varies between tree species, regionally and with local environmental and weather factors such as storminess, frost and temperature.
‘The University of Birmingham and a co-located UK Met Office team are working on a number of projects to help better observe the adhesion problem using the rapidly emerging internet of things. This approach takes advantage of new ubiquitous communications technology that enables widespread low-cost sensing and monitoring at an unprecedented scale. Better observations of the factors contributing to low adhesion improves the fundamental science knowledge, and ultimately improve forecasts of times and locations of low adhesion hotspots that can help target solutions. The Met Office team based at the University of Birmingham uses research to continually improve their leaf fall and low adhesion forecasting services, which are used by railway companies across the country to put timely and targeted mitigation in place.
There is a wide arsenal of mitigation techniques available, some of which are operational and others are novel. These include removing the contamination from the railhead using water jetting, citrus based solutions or even microwave technology. Any solution has the difficult job of removing the layer of residue, often likened to the non-stick coating on a frying pan, without damaging the rail itself, other railway safety equipment on the line, or the wider environment. The current solution in use on the cross-city line in Birmingham includes a fleet of four leaf-buster trains, fitted with high-pressure water jets, part of a national fleet of over 50. These are a lot more powerful than a domestic pressure washer and could, if left running in one place, actually cut through the steel rail. These trains run at regular intervals clearing the rails throughout the day. Many passenger trains use sand to help gain better traction during the autumn period. Drivers also have special training to help them accelerate and brake safely in slippery conditions, and of course, there are the notorious leaf fall timetables which help operators run a more resilient service by allowing trains more time to brake and accelerate. Altogether this means more trains running safely with the aim of fewer delays.
Looking forward, a significant unknown is how the adhesion problem will develop as the weather and climate continues to change. Warmer conditions may encourage vegetation growth with some species or perhaps affect the timing of fall so that leaves shed later. Alternatively, drier summer conditions may inhibit growth and encourage earlier leaf-fall in some drought intolerant species. The Birmingham Institute of Forestry Research (BiFor) is working to understand the impact of climate and environmental change on woodlands, work that is beneficial to the railway industry. Climate induced changes in temperature and rainfall patterns also affect the underlying railway infrastructure, weakening embankments and the ground underneath the railway. The University of Birmingham Centre for Railway Research and Education, in conjunction with the UK Met Office, are working on a variety of projects to support the rail industry in adapting both infrastructure and operational policies to cope and improve resilience. These include a new policy framework, supported by the International Union of Railways (UIC) to assist any railway transport company to develop a workable adaptation strategy and put it into action.
This integrates with the UK Climate Projections, launched on 26th November 2018, which provide the most up-to-date assessment of how the climate of the UK may change over the 21st century, and will help GB railways understand changes in risk and how they can adapt to be more resilient.