is the lubricant applied in minute quantities to the side surface of a train rail to minimize metal-to-metal contact between the wheel flange and rail flange. (The top of the rail must not be lubricated as this would impede the progress of the train.) The grease may be applied from an on-board lubricator through nozzles located just in front of the locomotive wheels, or from wayside lubricators located at rail curves, where wheel-rail friction is greatest. A rail-flange grease must have excellent anti-wear properties, water resistance, and adherence to lubricated surfaces.