Low Adhesion in the Wheel-Rail Contact

Adhesion, or adhesion coefficient, is given by the ratio of the longitudinal tangential (i.e., braking or traction) force over the normal force at the wheel-rail contact. The tangential force that a braking or tractive railway wheel can exert on a rail is limited by the friction coefficient available between the surfaces in contact for a given normal load. In clean steel-on-steel contacts, the friction coefficient is known to be mostly higher than the adhesion requirements for normal traction and braking operations of existing rolling stock. However, contaminations, such as leaves, grease and water, can easily be present at the wheel-rail contact and reduce the friction level, leading to low adhesion problems. In recent decades, some railways, such as in the Netherlands, the United Kingdom, Sweden and Germany, have particularly been affected in autumn due to the presence of moisture and fallen leaves, among other contaminants. When low adhesion occurs, delays in the train service may be the clearest consequence to the railway commuters. However, many other negative effects may arise, such as damages to wheels and rails, signals passed at danger, station platform overruns and even collisions. Therefore, not only the reliability but also the safety and costs of railway transportation may be compromised. Extreme low adhesion conditions in the Netherlands on October 27, 2002 forced the major train operating company (NS) and the infrastructure manager (ProRail) to stop the services on most of the sections of the network during that day and considerable disruptions continued the days after. In order to mitigate the low adhesion problem, the affected infrastructure managers and train operating companies have taken a variety of countermeasures. However, the problems still persist. This may partly be attributed to insufficient understanding of the problem and its possible countermeasures. The investigations presented in this dissertation, which have been commissioned by ProRail and NS within a research project called AdRem, have aimed at improving this understanding towards an effective solution to the problem. Four existing countermeasures have been investigated in this dissertation, namely friction modifiers, sanding, magnetic track brakes and traction control