Train ride quality plays an important role in train operation. In this paper, the aim is to explore the train ride quality when trains run on soft sub grade at different speeds especially critical conditions and operation safety on the sub grades with different properties. The responses of track and vehicle such as lateral wheel-rail force (L/V ratio), acceleration of vehicle are studied in this research. It is important to note that a critical condition is when train runs on soft sub grade. In this case, the critical speed effect will occur and resonance between vehicle and track could rise up the vibration level which could cause derailment and bad riding experience. In order to measure the track and vehicle responses, it should combine a detailed vehicle model and a track dynamic model. However, most commercial modeling programs do not have the ability to combine both detailed vehicle and track models. In this research, by integrating a computer program with a 2-D track model to a commercial vehicle dynamic program, the responses of vehicle and track can be calculated at the same time. Conclusions and findings will be illustrated in this paper.