•A rigid-flexible coupling vehicle dynamic was established with gear meshing excitation.•Teeth backlash and time-varying sliding friction were considered into torsional vibration model.•Internal excitation from traction units was included to ensure the operation safety and reliability.•Fatigue life was significantly improved by adopting S355J2W and weld toe polishing. Railway welded bogie is usually subjected to extremely complex fatigue loads, producing huge failure hazard. However, only external excitations from the wheel-rail system are considered when assessing the fatigue performance by most of the standards. Therefore, a novel approach for assessing the lifetime of a powered bogie frame considering internal excitation was proposed in this paper. To achieve this, a modified torsional vibration model considering gear meshing was first established. The calculated results were then input into a rigid-flexible coupled vehicle dynamic model to estimate the dynamic response and hence the cumulative damage was determined. This approach was applied to assess the lifetime of a weld polished bogie frame. The results indicate that internal excitation should be considered to determine the service lifetime.