Based on Biot's theory of porous media, this paper proposes a coupled vehicle-track-saturated foundation model to study the dynamic responses of saturated soft soil foundation under high-speed train load via two-and-half dimensional finite element method (2.5D FEM). It is found that the ratio of train speed c and soil Darcy permeability k_D(c/k_D) determines the extent to which maximum excess pore pressures build up in saturated soft soil under the train load when the load speed is lower than the critical speed. For a saturated soil of a particular stiffness, if c/k_D is less than or equal to 3×10⁴, the soil can be regarded as highly permeable relating to the load velocity. In this case, almost no excess pore pressure is developed, and the saturated soil can be modeled using a single-phase medium. There is a critical value of c/k_D for the development of excess pore pressure, corresponding to which the maximum excess pore pressure increases with increasing c/k_D. Above this critical value of c/k_D, the maximum excess pore pressure remains independent of c/k_D. The amplitude and influencing zone of effective stress are mainly controlled by the train speed and soil permeability for saturated soft soil foundation. The response of displacement is mainly controlled by the train speed. Significant Mach effects have been induced in the saturated soil foundation by the moving train when the train speed reaches or exceeds the critical velocity of the track-foundation system.