Many railways were constructed in regions of China that experience seasonal freezing and thawing cycles in subgrade soils. Consequently, the dynamic stress distribution in subgrade soils induced by passing trains was observed to vary in different seasons. The research first focused on the development of a vertical train-track-subgrade coupled dynamic model, which considered the vibration of soil layers and the time histories of loading. Second on the effect of seasonal variations on the dynamic stress in subgrade soils, which was conducted by FEM computational simulations. Based on the results, the dynamic stress distribution and amplitudes were closely related to the frozen or thaw states of the subgrade soil. Third on the development of a simplified method for calculating the dynamic stress in subgrade soils induced by passing trains, which could be used in railway subgrade designs in seasonal frozen regions. The achievements of this research will be useful for optimizing the subgrade design method and completing the evaluation method of the long-term stability of the subgrade soil.