Principles and results of application of specially designed ultrasonic flaw detector for detection and evaluation of parameters of transverse cracks, including under surface damages of rail head, are considered. Using a fairly simple method of multichannel shadow method of ultrasonic inspection and special algorithms for digital signal processing, a flaw detector scanner was developed and introduced, which allows layer-by-layer scanning of a defective section of the rail to visualize a defect in three orthogonal planes. Multiple comparisons of the obtained flaw detection images with the results of forced dolomes show that the measurement errors of the proposed method do not exceed 15%, which is a very high indicator for practice. Proven principles are recommended for use in other industries. Keywords: ultrasonic flaw detection, rail head, size measurements of cracking, through sonic test, shadow method.