In some example implementations, there is provided a method. The method may include generating, by an air-coupled transducer, a first ultrasonic guided wave to cause the generated ultrasonic guided wave to propagate into a rail being tested for one or more defects, wherein a frequency of the first ultrasonic guided wave is controlled by at least changing the frequency of a voltage sent to the air-coupled transducer generating the first ultrasonic guided wave; receiving, by a receiver, a second ultrasonic guided wave, wherein the second ultrasonic guided wave is received from the rail; and analyzing a signal representative of the received second ultrasonic guided wave to detect the one or more defects in the rail. Related systems, methods, and articles of manufacture are also provided.