Abstract: The primary parameters that govern the dynamic factors of a simple-supported beam traversed by discrete moving loads are analytically identified. The concept of necessary mode amount is proposed to achieve the required accuracy for the dynamic responses of various locations on the beam. Thus the principle is established to determine the amount of modes of vibration required in the modal superposition based dynamic factor analysis. A train consists of eight vehicles is modeled as three types of common used discrete moving load models for comparison. The differences between dynamic factors obtained from the three simplified load models are evaluated regarding a simple beam with span length of 20m. It is found that the moving load model representing the actual locations of the axles of the train should be used for quantitative analysis of dynamic factors.