Abstract: The dynamics performances of NX70 and NX70A with different truck center length were studied. The virtual models were built in Simpack. The running and loading conditions were designed in accordance with the railroad line and loading technical specification in China. Derailment coefficient and wheel unloading rate were used as evaluation indexes. The peak value and RMS of indexes were calculated for evaluating dynamics performance. The simulation results indicate that the flat car with longer truck center has better dynamics performance.