Loss of adhesion between a railway wheel and the track has implications for both braking and traction. Poor adhesion in braking is a safety issue as it leads to extended stopping distances.In traction it is a performance issue as it may lead to reduced acceleration which could cause delays.In this work wheel/rail adhesion was assessed using a twin disc simulation. The effects of a number of contaminants, such as oil, dry and wet leaves and sand were investigated. These have been shown in the past to have significant effect on adhesion, but this has not been well quantified.The results have shown that both oil and water reduce adhesion from the dry condition.Leaves, however, gave the lowest adhesion values, even when dry. The addition of sand, commonly used as a friction enhancer, to leaves, brought adhesion levels back to the levels without leaves present. Adhesion levels recorded, particularly for the wet, dry and oil conditions are in the range seen in field measurements.Relatively severe disc surface damage and subsurface deformation was seen after the addition of sand. Leaves were also seen to cause indents in the disc surfaces.The twin disc approach has been shown to provide a good approach for comparing adhesion levels under a range of wheel/rail contact conditions, with and without contaminants.

Leaf-related adhesion problems have been present in many railway networks all over the world in the last few decades. Since the early 1970s many measures have been undertaken in order to mitigate the problem. One of the measures adopted by many railway networks is the use of friction modifiers. However, the low adhesion problem still persists. Furthermore, the effectiveness of these friction modifiers has not well proven yet due to the lack of research in controlled conditions. Consequently, the rolling stock operators and infrastructure managers do not clearly understand the performance and side effects of the friction modifiers used on their networks. In this paper, an investigation of the performance of two existent friction modifiers in controlled laboratory conditions is presented. These friction modifiers have been used or tested in several railway networks. A twin-disk roller rig has been used to study their performance in leaf contaminated contacts. The adhesion characteristics of both friction modifiers are examined for different slip ratios. The constituents of the friction modifiers are identified and the solid components are analyzed. In addition, damage that these friction modifiers may cause to wheel and rail is also discussed.

Leaf contamination has been identified as the major cause of low adhesion incidents occurring on some railway networks in the last few decades. In the presence of leaf layers, the trains cannot have the required adhesion at the wheel—rail contact for adequate traction and braking operation. Under these circumstances, not only the punctuality but also the safety of the railway transportation can be threatened. In order to mitigate low adhesion problems, railway organizations have opted for different measures, particularly during the season of Autumn. The most employed measure consists of bringing sand to the wheel—rail interface, which can be performed by means of air-pumped sanders or in the form of sand-based friction modifiers. Although sand has widely been accepted as an effective adhesion improver, the effect of some sanding parameters on the adhesion improvement in leaf-contaminated contacts seems to be unclear. This hinders the possible optimized use of sand on the railway networks. In this paper, the influence of the number of sanding axles, particle size of sand, and wheel slip on the adhesion recovery in leaf-contaminated wheel—rail contacts is presented. Rolling—sliding tests under closely controlled conditions have been performed on a twin-disc roller rig. An electrical circuit has been connected to the rig for monitoring the effect of contamination on the electrical conductivity across the wheel—rail contact. The results show that the application of sand contributes to removing the leaf layers from the disc surfaces, which leads to a higher adhesion coefficient in comparison with the untreated (baseline) situation. Accordingly, the electrical conductivity across the wheel—rail contact is also improved. Furthermore, the adhesion recovery is shown to become larger and faster with the increase in sanding axles and wheel slip. Among the particle sizes tested in this work, medium particles are found to yield the most effective adhesion recovery.

A popular practice is the application of friction modifiers to increase the adhesion level between wheel and rail under different contamination conditions.Particularly, two friction modifiers have been used or tested in several railway networks as adhesion enhancers to facilitate the traction and braking operation under poor adhesion conditions. However, the railway operators and infrastructure managers only count with practical observations that do not elucidate completely the effectiveness and side effects of these adhesion enhancers. In this paper, a twin-disk roller rig has been used to study their performance in dry and wet contacts under closely controlled laboratory conditions. The adhesion characteristics of both friction modifiers are examined for different slip ratios. The constituents of the friction modifiers are identified and the solid components are analyzed. Furthermore, the wheel and rail disks are examined after a series of dry tests to analyze the mass loss, surface damage, modification of surface hardness and roughness, and subsurface deformation caused by the friction modifiers compared to dry contacts.

This article details the findings of a series of twin disc machine adhesion tests that investigated the effects of oil and water mixtures on adhesion at the wheel/rail interface. Oil was found to have a dominant effect on adhesion in the presence of water. Surfaces coated with oil at 4.7×10−3 g/cm2 and sprayed with water had levels of adhesion similar to those coated by a replenishing supply of oil. The tests showed that drying a wet contact can initially give a reduction in adhesion, that increased roughness results in increased adhesion in the presence of oil, and that increased contact pressure improves adhesion in the presence of oil.