<p>Nonlinear ultrasonic frequency mixing exhibits high sensitivity and good localization capability for closed cracks. In this paper, nonlinear ultrasonic frequency mixing technology is used to investigate the contact acoustic nonlinearity phenomenon in turnout rails. The influence of different frequency mixing mode pairs and crack sizes on the frequency mixing signal is studied by selecting mode pairs suitable for detecting cracks at the rail bottom. The results indicate that the interaction between guided waves and the crack can generate frequency mixing signals. At the mixing frequencies, multiple guided wave modes will be produced, and the mixing signals increase significantly with the size of the cracks. This demonstrates the damage detection capability of nonlinear ultrasonic frequency mixing for closed cracks, providing a basis for damage detection in turnout rails.</p>