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 result of low adhesion is a number of problems:
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 grip, or slipperiness, between the wheel and rail. We can measure adhesion levels and a value of adhesion is assigned normally expressed as 'μ' (a decimal fraction) or sometimes as a percentage.
The graph below, produced from actual surveys undertaken in the 1990s by the British Rail Tribometer train, shows how frequently the various adhesion levels occur on the railway. It can be seen that the exceptionally low levels of adhesion (below 5%) are rare as are the very high levels (above 35%). For most of the time the available adhesion levels are well within those required to sustain normal braking and traction power demands.
Low adhesion is infrequent - autumn mornings
Very low adhesion is very infrequent - mostly damp leaf