<p>As a core load-bearing component of railway systems, heavy rail steel is prone to crack initiation induced by inclusions during service, which adversely affects its fatigue life. Owing to its excellent deoxidation, desulfurization, and inclusion modification capabilities, the rare earth element Ce is considered an effective approach to improving steel quality. A combined approach of thermodynamic calculations, high-temperature melting experiments, and crystallographic analysis was employed to systematically investigate the modification mechanism of inclusions and the heterogeneous nucleation behavior induced by Ce in heavy rail steel. The results show that the primary inclusions in untreated heavy rail steel are large, irregular MnS particles and nearly spherical Al₂O₃–SiO₂–CaO complex inclusions. After Ce addition, Ce reacts not only with free O, S, and Al in the molten steel but also modifies the existing inclusions into smaller, more spherical or ellipsoidal CeAlO₃, Ce₂O₃, Ce₂O₂S, and their composite forms. Due to the better toughness of Ce₂O₃ and Ce₂O₂S, when they envelop the harder CeAlO₃ core, they help alleviate stress concentration at the interface with the steel matrix, thereby improving the mechanical properties of the steel. With the&#xa0;increasing Ce content, the inclusion size shows a trend of first decreasing and then increasing. The optimal modification effect is observed when the Ce content is in the range of 0.0017%–0.0026%. The overall evolution sequence of inclusions is as follows: Al₂O₃ + MnS→ CeAlO₃ + MnS→ Ce₂O₂S + MnS→ Ce₂O₂S + Ce₂S₃→ Ce₂O₂S + Ce₃S₄ + CeS→ Ce₂O₂S + CeS. Furthermore, the two-dimensional lattice disregistry analysis reveals that rare earth inclusions exhibit better lattice matching with γ-Fe than Al₂O₃, indicating stronger heterogeneous nucleation potential. During solidification, Ce₂O₃ and Ce₂O₂S tend to nucleate on CeAlO₃ surfaces to form complex inclusions, which in turn provide nucleation sites for γ-Fe and promote grain refinement.</p>