The Alliance Performance Improvement Team anaylised past incidents which showed that the likely cause of 60 percent of station overruns and other performance issues were related to wet rail conditions. The majority of these over runs were by less than half a coach length or less than 10 meters.
Existing wet rail mitigations such as high pressure jetting and traction sand are not totally effective at improving adhesion under these conditions.
The railways have been operating with the same basic steel wheel on steel rail interface for over 200 years. This steel on steel interface was adopted not only because of its strength and low wear properties, but also because it offers a low rolling resistance thus reducing considerably the effort required to move heavy loads.
However, this advantage sometimes becomes the railway’s Achilles heel, particularly during the autumn leaf fall season. During this time of the year, but not only at this time, the rail surface and the wheel treads can become coated with a range of contaminants.
The worst of these are crushed leaves, which, when combined with moisture, particularly in the form of dew or condensation, reduces the adhesion level. For a train on dry rails adhesion is typically around 0.25, on wet rails it is around 0.15, but on damp leaf it can be as low as 0.015. Rails with damp leaves significantly constrain the rate of braking. Furthermore, it can also have a profound effect on train performance because low adhesion jeopardises acceleration as well.
Leaf related problems are not new and have been encountered for decades. Immediately after the demise of steam traction operation in the late 1960s, vegetation control was reduced allowing the lineside to sprout into ‘linear forests’. As a result, the low adhesion problems got worse over time and it became necessary to re-instate high levels of lineside vegetation management. However, it is not always possible for the railway to manage all trees as they are not always on railway property.
Further, with the advancement of technology and changing train operation, more demand has been placed on higher adhesion levels to support higher braking rates, shorter yet faster trains and more frequent services. The nature of the difficulty encountered depends on a vast range of factors which change constantly.
The ‘adhesion profile’ along any stretch of line varies within metres: the temperature and humidity levels can change rapidly; contaminants react differently to the passage of a train; the trains are driven differently; the trains themselves are different, and so on.
Low adhesion occurs all year round, not just in the autumn. Wet rails, accompanied by rail-borne contaminants, can offer low adhesion levels despite the rails looking clean. Analyses have shown as many station overrun incidents due to poor rail conditions can occur outside of the autumn period as during it.
The annual cost of low adhesion to the GB rail industry as a whole has been estimated to exceed £100 million. This arises from many different causes, some of which are difficult to quantify:
This of course does not include the significant consequences of a serious incident such as a collision or derailment, which could occur as a result of increased braking distances or a failure of a track circuit to detect the presence of a train in a section.
Adhesion on the railway, put in simple terms, is a measure of the Traction, or slipperiness, between the wheel and rail.
In dry weather with clean (shiny) uncontaminated rails, the adhesion level would commonly be found to be between 20% and 40%, in really wet conditions it may be between 10% and 20%.
The Rail-Traction wet rail improver formulation has been developed and tested over a number of years in conjunction with South West Trains and the Network Rail Alliance Team who were tasked in finding a solution to mitigate the effect of low adhesion on the railway network.
Download the Rail Traction product brochure and technical datasheet.
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