Understanding post-derailment behaviours is critical and fundamental to preventing derailment escalation and developing containment methods, thereby avoiding catastrophic consequences in a derailment accident. However, modelling and simulating the train post-derailment behaviours is a significant challenge because of the complexity and unpredictability of the train post-derailment contact interaction. This paper presents a novel post-derailment contact model built upon the Polygon Contact Model (PCM), aimed at enabling accurate and efficient simulation of train post-derailment contact interactions. Expanding upon the PCM-based post-derailment contact model, we introduce a framework for simulating train post-derailment behaviours and utilise it to develop a train post-derailment dynamics model that considers complex contact interactions between the bogie and the track structure. Additionally, we introduce a dynamic optimisation method to determine the essential parameters for this model. The feasibility and accuracy of the PCM-based train post-derailment dynamics model have been validated through a comparative study with an actual derailment test. Subsequently, the verified model is used to reproduce different post-derailment scenarios with varying combinations of vehicles and tracks, yielding valuable insight and understanding into the train post-derailment behaviour.