Fouling refers to the condition of railroad ballast when voids in this unbound aggregate layer are filled with fine materials or fouling agents commonly in the form of ballast aggregate breakdown, outside contamination such as coal dust from coal trains, or from subgrade soil intrusion. Effects of coal dust on shear strength of different ballast aggregates were studied through the use of a large shear box device. The strength properties of both granite and limestone ballast samples were determined when coal dust was added to clean ballast samples at various percentages by weight of ballast under both dry and wet conditions. When the coal dust fouling percentage increased, the ballast shear strength generally decreased. Wet fouling was found to exacerbate this trend. However, coal dust had more detrimental impact on granite ballast strength when compared to limestone ballast. This could be caused by limestone's higher crushing tendency and/or possibly any differences in void structures since gradation properties of the granite and limestone samples varied.