The majority US high speed trains are sharing the same track network with freight trains, which will bring different load patterns i.e. different peak loads, speeds, and loading sequence and durations to the current track infrastructure. The change in loading characteristics especially when train is moving at so called "Critical Speeds" may however cause unfavorable track performances, for instance increase in track settlement (both in ballast and the subgrade soil). Effort has been taken in this research to investigate the impact of different load patterns on the track settlement accumulation. A fully coupled dynamic track model consists of a vehicle, rail-tie-ballast "Sandwich" upper structure, and a finite 3D soil substructure model is developed and utilized to obtain the loading pulses and pulse combinations introduced by different traffic patterns. A Discrete Element Modeling approach is then employed to simulate the ballast settlement under those realistic loading pulses and pulse combinations. Results and conclusions will be highlighted and recommendations will be made in the end.