Abstract: An ultrasonic rail web centerline detector locates the web centerline (18) of a longitudinal rail (12) having a head (80), a base (82), and a web therebetween. The detector includes an ultrasound sensor (14) acoustically coupled to the head (80) of the rail (12) for transmitting ultrasound energy through the rail (12) and for generating electrical signals corresponding to received reflected ultrasound energy. A controller (16) responsive to the electrical signals generates control signals representative of a lateral and angular displacement of the ultrasound sensor (14) with respect to the web centerline (18) of the rail. The position of the ultrasound sensor is adjusted as a function of the control signals representative of the lateral and angular displacement. The ultrasound sensor includes a center transceiver (30) and side transceivers (32, 34) which are acoustically coupled to the head (80) of the rail (12).