Vibration response of track and foundation subjected to dynamic loading is one of the key issues to solve on-track safety of high-speed train. The previous pioneering works commonly only considered the train moving load, however, in reality, trains are likely to be on track when an earthquake occurs due to the high frequency and widespread distribution of earthquake activities. Hence, a three-dimensional FEM of track-subgrade- foundation interaction system with bidirectional seismic and moving loads is established for investigating the differences and relations of vibration responses of subgrade in such two immensely disparate loads: train moving load and earthquake-moving load. As a case study, the vibration characteristics of the Beijing-Shanghai High-speed Railway of the China, excited by moving load and seismic-moving composite load are analyzed respectively, with various velocity (v = 50 m/s, 70 m/s, 100 m/s, 130 m/s). On the other hand, the increases in operational train speeds mean that critical velocity effects are becoming more common on high speed rail lines. If unaddressed, critical velocity issues can cause safety concerns and elevated maintenance costs. Based on the derailment coefficient and lateral deformation of the rail, the critical speed of the model is discussed, which is a reasonable improvement to the derailment mechanism of the train.