Electric traction transformers are core components in electrified railway systems, converting high-voltage grid power to low-voltage power for traction motors. Operational, manufacturing, installation, or maintenance issues can cause unequal turns in high-voltage windings, leading to increased circulating currents, higher losses, magnetic field distortion, and other adverse effects, compromising transformer safety. This study focuses on a 25 kV single-phase two-winding traction transformer. A three-dimensional finite-element model was developed using ANSYS Maxwell. The model was validated through no-load and short-circuit tests, with less than 5% error in key parameters, and was used to systematically examine the effects of high-voltage winding unequal turns on the transformer’s electrical parameters, magnetic field distribution, and electromagnetic forces. Based on the analysis, a diagnostic method for detecting winding unequal turns using potential difference measurements is proposed. Simulation and experimental results demonstrate the efficacy of the proposed approach in identifying unequal turns faults.