Although automated testing instruments have been introduced intensely on the railways of Russia, up to 95% of hazardous flaws in rails are detected by ultrasonic two-line (removable) flaw detectors. The paper describes the functional diagram and basic functions of an ultrasonic detector of the new generation, AVIKON-01, with continuous recording of data. The instrument is supplied with the software package for analysis of testing results, and it has advantages of both hand-held removable flaw detectors and high-performance instruments for nondestructive testing of rails.