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This paper presents a transient-dynamic method (TDM), to investigate the dynamic responses of a railway formation under a train moving load using a three-dimensional finite element model. First, the feasibility of the TDM was verified, by comparing the vertical stress in a railway formation calculated using this method with the steady-state method (SSM). Next, the effects of train speed and embankment stiffness on the dynamic response of the railway formation were evaluated using TDM. The numerical results indicated that the vertical displacement/stress of the railway formation were remarkably increased with an increased train speed, within the shear wave velocity of the soft soil. In contrast, the vertical displacement/stress attenuation with the depth of the ground caused by high-speed trains was faster than that caused by low-speed trains. As for the effect of embankment stiffness, the enhancement of embankment stiffness had little effect on the transfer of vertical stress to the underlying soil. Finally, a determination of the dynamic stress for different train speeds was evaluated. This design code may underestimate the dynamic stress in a railway formation in high-speed situations (e.g., <i>v</i> = 112 m/s).