The aim of this paper was to review the current state of research for top of rail friction modifiers (TORFM). In the railway industry, friction modifiers is a catch all term for a wide range of products applied for different purposes which has led to confusion. It is hoped that recently published definitions will aid industry to a better understanding of the different products and how they function. The benefits of friction modifiers are well understood with a large body of research supporting the benefits. Comparatively, there is a lot less knowledge of the optimum amount of product to achieve the benefits or how far down the track from an application site the benefit will be seen. Modelling of the products is another area where there is little research, with most of the modelling papers found focussing on dry wheel–rail contact due to the complexity of introducing a third-body layer to a friction force model. Furthermore, only one paper was found which relates how friction modifiers are affected by contaminants or other applied products such as lubricants. With many different products applied to wheels and rail for different purposes, understanding their interaction is key. At the time of this review, there are currently no standards that prescribe how TORFM should behave although the European Committee for Standardisation is currently developing them at the moment. This review has also attempted to appraise the research against a set of criteria. Depending on how many of the criteria the piece of research filled, it was categorised as A, B or C. It was found that most of the research was of category, this was mainly due to only one test method being used or the scale presented. Category A research incorporated modelling or multiple test-scales to support the results presented.
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