In United States railway history, a whistle stop is a station which a train stops at only on request, as when there are passengers or frieght to be taken on or discharged. Ordinarily, a flag or other visual indicator would be placed outside the station by the station master if the station had traffic. If a stop was requested by a passenger on the train, a whistle would be sounded by train personnel to alert the locomotive engineer to the need to stop.
