Abstract: An ultrasonic rail flaw detection system and technique are described for the detection of flaws in the sides of rail heads. An ultrasonic transducer is oriented, sized, and so-located so as to produce an ultrasonic beam within the head whereby at least a portion of the beam will intersect a lower side corner independent of the curvature of the rail head running surface and the amount of head wear. The presence of premature reflections or the or absence of reflections can be used to detect flaws such as vertical fractures in the rail head. The side-looker is so oriented as to produce a beam in the rail head that lies within 45.degree. to 75.degree. of the normal to the rail surface. The side-looker can further be so located that its beam does not interfere with or be interfered by beams from other transducers used to investigate the rail and mounted in the same wheel assembly as the side-looker.