A possible virtual dynamics method is developed to replenish the wheel-track contact force histories for assessing properly the vehicles dynamics-fatigue properties under unknown real railway line spectrum. The work starts from known partial on-line inspection data of the force histories and the two known referenced line spectra, i.e. American railway line spectra of grades V and VI. The inspected data are used for selecting a closer line spectrum to real one from the two known spectra, and then, for verifying the developed method. The closer one is determined by comparing the predicted force histories by the two referenced spectra to the inspected data, respectively. The simulation method is gradually developed by adjusting vehicle’s dynamics parameters in terms of a virtual dynamics method to reduce the difference between on-line inspected data and the dynamics simulations. Availability is verified by the method developing process on dynamics simulation of China railway freight car with 23 ton axle weigh.