Abstract: An apparatus for calibrating a rail scale is described. The apparatus is attached to a first trailer and a second trailer and the move from a first site to a rail scale site over highways and roads. The apparatus includes a calibration vehicle, bins of weights, and a crane. The crane positions the calibration vehicle, that is self-propelled, on rails and loads weights on the calibration vehicle. The combined weight of the calibration vehicle and weights has qualifying values that ensures a scale is in compliance rail scale calibration standards.