An apparatus and method for accurately weighing a rail car coupled to other rail cars without uncoupling the rail car to be weighed includes a hydraulically operated positioning device assembly with opposed pushers that can be elevated to hold the axle of a rail car uptrack of the car to be weighed, a scale mounted downtrack of the rail car positioning device, and a car stop mounted downtrack of the scale that can be elevated to block the wheels of a car downtrack of the car being weighed.The rail car positioning device engages the car uptrack of the car to be weighed, moves the car to be weighed over the scale, and, by reversibly moving the car, centers the car over the scale. The car stop then engages the downtrack car to maintain it and other cars coupled to it downtrack in a fixed position during weighing and filling of the car on the scale. The carriage assembly then releases the car on the scale and moves uptrack where it acquires a hold on the next empty car. With the downtrack cars from the car to be weighed on the scale immobilized by the car stop, and the uptrack cars held by the reversible rail car positioning device, the uptrack cars can be moved back and forth to relieve all coupling stresses from the car to be weighed. This back and forth movement can be repeated at any time during the loading cycle to assure that coupling stresses do not influence the accuracy of the final measured cargo weight.