To address the difficulty in identifying echoes from near-surface railhead defects caused by near-field interference, this paper establishes an ultrasonic sound-field model in rails based on the Huygens integral, and combines numerical simulations and experiments to investigate the effects of probe frequency and piezoelectric element size on the sound-field distribution and defect-echo features. The results show that, as the element size increases from 10×10 mm to 20×20 mm, the near-field length increases from 11.9 mm to 51.4 mm; as the frequency increases from 1 MHz to 4 MHz, the near-field length increases approximately linearly, while the peak sound pressure decreases overall with increasing frequency and element size. In the near field, defect echoes overlap significantly with the initial pulse, making near-surface railhead defects difficult to distinguish; in the far field, the echo separation improves and the detection accuracy increases markedly.