Top-of-rail (TOR) lubricants are commonly used for friction control in railway operations. They aim to lower friction and reduce noise and wear while ensuring sufficient transmission of traction/braking forces. However, the wheel–rail interface is an open system, so the conditions may suddenly change due to the weather, and different contaminants may enter the contact and influence the performance of these lubricants. Thus, this study examined the effect of humidity and dew on two commercial products, as these conditions often occur on the track. A methodology based on a creep curves measurement approach was used to assess product performance under various scenarios. All measurements were conducted on a universal tribometer in the ball-on-disc configuration covered with a climate chamber. The results show a strong influence of dew on the tested products, as dew lowered their performance parameters and caused low adhesion problems. Possible mechanisms of water–oil interaction and formation of oxidic third body layers were discussed. The main findings indicate that TOR lubricants may cause traction/braking problems if used in dew conditions. The present study may be helpful in optimising friction management methods in the future. Graphical Abstract