In this thesis, a multi-fluid, multiphase, numerical model was developed to investigate the flow field, heat transfer and solidification process during the AT welding of rails. The focus of the study was on understanding the thermal flow field, solid-liquid phase change during the pouring stage, and the temporal evolution of the solid-liquid interface front during the solidification process. The numerical investigations were performed using the FVM-based open-source CFD code OpenFOAM. The multiphase flow solver, interFoam, for simulating two incompressible immiscible fluids using the VoF method was modified to incorporate the energy equation and predict the heat transfer within the system.