The present specification discloses a high speed scanning system for scanning cargo carried by rail. The system uses a two-dimensional X-ray sensor array with, in one embodiment, a cone-beam X-ray geometry. The pulse rate of X-ray source is modulated based on the speed of the moving cargo to allow a distance travelled by the cargo between X-ray pulses to be equal to the width of the detector, for a single energy source, and to half the width of the detector for a dual energy source. This ensures precise timing between the X-ray exposure and the speed of the passing object, and thus accurate scanning of cargo even at high speeds.