Abstract: An air flow sensor is disposed in the main supply line of a train air brake system. Circuitry is connected to the sensor and initiates the incrementing of a counter upon the sensing of an air flow rate above a predetermined level for greater than a predetermined length of time. The counting is terminated upon application of the locomotive brakes. The counter thereby produces an indication of elapsed time between the time of an air brake leak occurrence and the time the decrease in pressure is sensed by the locomotive. This indication can be converted to feet based upon speed of travel of the pressure head and divided by the average length of cars in the train to provide an accurate estimate of the location of the leak.