Keywords: wheel-rail interaction, friction modifier, flange lubrication, twin-disc testing, wheel–rail contact, enviromental influence.

The performance of top-of-rail friction modifiers (TOR-FMs) is critical for balancing traction, braking, and wear in railway systems, yet is strongly influenced by environmental conditions. This study evaluates the effect of temperature and humidity on oil-based TOR-FMs in Swiss metre-gauge railways using a high-load twin-disc tribometer equipped with a custom climate chamber. Wheel and rail specimens were subjected to controlled winter, summer, and wet/snow-like conditions, while friction coefficients (CoF) and film stability were monitored. Results show, that low temperatures (?0°C) and high humidity (90 %) produce low and stable CoF (~0.10 – 0.15) over extended cycles, whereas increasing temperatures (15 – 35°C) and lower humidity (25 – 60 %) substantially reduce TOR-FM durability, accelerating the rise of CoF. The study further highlights the importance of precise sensor placement and climate chamber design in capturing realistic wheel-rail interactions. Findings validate current Swiss operational strategies, such as suspending TOR-FM application above 80 % humidity, and suggest optimized application strategies under high-temperature, low-humidity conditions to enhance traction, reduce wear, and mitigate noise effectively. This work underscores the critical role of environmental factors in TOR-FM effectiveness and provides a framework for systematic laboratory evaluation under representative weather conditions.

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