With a view to examining the wheel-rail impact forces due to track geometry irregularties, the simulations of wagon-track system dynamics have been carried out and are presented in this paper. Two types of Pacific National (PNL) wagons (120-ton coal and 79-ton container) containing new and worn wheels, and measured track geometry irregularities supplied by The Australian Rail Track Corporation (ARTC) have been used in the simulations. lt is shown that the alignment irregularity contributes the most to the lateral impact whilst the vertical surface profile irregularity contributes the most to the vertical impact for theses specific measured track irregularities. lt is shown that the wagons with severely worn wheels could lead to wagon derailment. It is also shown that the increase of wagon speed generally increases the impact, and the lighter wagon is more sensitive to the increase of wagon speed.