Settlement will occur at subgrade of high-speed railroad caused by repeated loadings from train traffic. Train’s operation speed has a direct correlation with the settlement of railroad subgrade. As train’s running speed increases, the criteria for controlling differential settlement of railroad subgrade becomes higher. Meanwhile, the further increase of train velocity is limited by the subgrade’s differential settlement. It is therefore necessary to investigate the influence of subgrade settlement on the vibration of train and track structure. An updated numerical model is developed based on traditional vertical train-track coupling theory, in which the CA mortar and concrete base beneath slab track are both taken into account. The calculated and in-situ tested results are compared to validate the model and the computation program. Using the program, the amplifications of train-track interaction under different conditions of roadbed settlement and train velocities were analysed. It is concluded that while short-range roadbed settlement result in higher axle acceleration, car body vibration does not increase obviously due to the high frequency of vibration. Therefore, amplitude of settlement should be controlled under the safety standard (axle load reduction rate). In the case of long-range roadbed settlement, however, car body vibration increases obviously. Amplitude of settlement should therefore be controlled under the comfort standard (car body acceleration). The specific control parameters are given at the end.