A generation method for the Rail random irregularity with arbitrary wavelength interval (WI) is developed, and its accuracy and efficiency are demonstrated. Then a moving wheel-Rail-bridge interaction element is derived to establish the finite element equations of motion for the train-Railbridge interaction system, and the flow chart of assembly and calculation for the system equations is given. According to the sub-interval principle, the influences of the irregularities with the large WI and the sub-intervals on the dynamic responses are analyzed by a numerical example, and the sensitive WI of each response is discussed. The results indicate that the bridge acceleration and the contact force are both more sensitive to the irregularity with WI (1~5 m). The irregularity with WI (0.1~1 m) has less influence on the car body acceleration but mainly contributes to the Rail acceleration. However, all the irregularities with wavelengths at interval (1~150 m) can have significant influences on the car body acceleration. Meanwhile, the transient jump of wheel can be simulated and should be taken into account for the deRailment risk assessment.