Fouling refers to the condition of railroad ballast when voids in this unbound aggregate layer are filled with relatively finer materials or fouling agents commonly from the ballast aggregate breakdown, outside contamination, or from subgrade soil intrusion. When ballast gets fouled, this unbound layer may not adequately serve its primary functions to provide drainage and structural support under the repeated loading applied by trains. Coal dust fouling has been identified as one of the most critical fouling scenarios as it significantly decreases the strength and stability of railroad ballast. A Discrete Element Modeling (DEM) approach was adopted in this study to investigate the coal dust fouled ballast performances. A DEM program BLOKS3D was used to employ realistic three-dimensional polyhedrons as discrete elements generated through digital imaging of aggregate particles. An appropriate DEM model parameter calibration process was carried out to predict both the clean ballast condition and the partially to fully coal dust fouled ballast behavior as obtained from laboratory shear box tests, which properly validated the BLOKS3D DEM simulations.