Smoothness during heavy haul train (HHT) running in the railroad has always been an important factor affecting the safety of HHT operation. With the increasing length of HHT formation, more and more optimization problems for the speed trajectory of HHTs start to take the improvement of train operation smoothness as the optimization objective. However, using the existing train dynamics model as the basis for calculating the running state of HHTs often faces the problems of being unable to characterize the longitudinal impulse or the required calculation time is too long. Therefore, this paper proposes a model improvement method based on the two main train dynamics computational models currently in use, in order to obtain the longitudinal impulse while calculating the running state of HHT quickly, which could provide a basic solution model for the subsequent optimization of the speed trajectory of HHT. Finally, several simulation scenarios are designed to verify the effectiveness of the proposed method.