The hook tongue parts of railway locomotives undergo squeezing, flexing, and impaction over long time periods, causing material fatigue. Cracks in railway locomotives gradually expand over the long term, which can cause serious accidents. Thus, it is very important to detect such cracks. However, as the surface of a railway locomotive hook tongue is covered with rust, dust, oil, etc., conventional nondestructive testing technology cannot be applied directly to it. Ultrasonic thermography technology was used in this study to detect cracks in the coupler parts of locomotives. The technology was not sensitive to the shape of the surface and the rust, dust, and the other pollutants on it. The technology offers special application advantages in the detection of cracks and other defects. Crack flaws were detected successfully during the crack detection for a locomotive hook tongue, thereby verifying the detection ability of the technology.