In response to the tracking control challenges faced by freight trains under multi-source disturbances, this paper proposes a fixed-time replacement sliding mode control method based on a cascaded disturbance observer. Initially, a multi-mass dynamic model that takes into account the inter-vehicular forces is constructed. To address the scenario where both matched and unmatched disturbances coexist, a cascaded structure disturbance observer is designed to estimate multi-source disturbances concurrently, thereby relaxing the traditional prerequisite of disturbance observers that dictates disturbances must vary slowly. Utilizing the disturbance observation data, the train dynamics model under unmatched disturbances is transformed into a matched mode. Ultimately, a distributed control strategy based on the alternative sliding mode approach is put forward. Simulation results demonstrate that the proposed cascaded disturbance observer can accurately estimate multi-source disturbances within 0.5 seconds. Compared with traditional research on train tracking control, the proposed control strategy manages to swiftly handle a series of destabilizing issues caused by multi-source disturbances. While ensuring the stability of inter-vehicular forces, it achieves robust tracking control of both speed and displacement indices. Relative to conventional control methods, the system convergence time is enhanced by more than 5 seconds, reflecting superior real-time performance and robustness.