An apparatus for detecting defects in a railway rail mounted on a test vehicle. The apparatus includes a transducer assembly with one or more arrays of ultrasonic transducers directed toward the running surface of the rail. Beams transmitted by the one or more arrays of ultrasonic transducers may be dynamically adjusted to compensate for the varying profile of the rail head and running surface. A laser profiler mounted on the test vehicle in combination with a linear encoder provide profile data, which is communicated to a system controller to dynamically adjust the focal laws for the one or more arrays of transducers to steer the transmitted beams to produce the ideal inspection beam sets while the test vehicle is in motion.