This disclosure relates to a predictor for determining the coefficient of friction existing between the rotating wheels of moving railway vehicles and the brake shoes of a pair of brake elements prior to the humping operation in a classification. One of the brake elements is slidably mounted to move longitudinally relative to the track rail when engaged by the rotating wheels of the railway vehicle. The kinetic energy or force imparted to the slidable brake element is transposed in fluid pressure by a pressurized responsive transducer. The fluid pressure is sensed by a measuring device which activates an alarm when the wheels of the railway vehicle are dirty or contaminated by unctuous material to permit the vehicle to be diverted to a sidetrack for special handling.