In mountainous regions where traction loads constitute a significant portion of a long-chain weak power grid (PG) with a high penetration of sustainable energy sources, the interaction between the traction power supply system and the PG becomes increasingly significant. The integrated power flow calculation (PFC) method and its linearized model are critically important for the PG-traction network (TN) joint planning. However, existing research on the port-load characteristics of EMUs and the connection-angle characteristics of traction transformers remains insufficient, and no effective methods for PFC or linearized PFC exist in systems that couple the PG to the traction network. To fill this gap, this paper proposes an integrated PFC model for the AT TN − PG coupled system and a corresponding linearized method. First, based on the phase relationships between the PG and the AT traction network, the node admittance matrix of the coupled system is constructed. Then, the inability of power injection equations to handle the EMU port load is resolved by merging the contact line node and the rail node. Subsequently, the integrated PFC equations for the coupled system are established. Next, a hybrid phase linear decoupled (HPLD) power flow model for the coupled system is developed by leveraging the phase correspondence between the PG and the TN and the phase angle differences across various nodes and branches. Numerical simulations conducted in a specific region demonstrate the necessity of an integrated PFC for the coupled system and validate both the accuracy and efficiency of the linearized model.