The railroad ballast layer consists of discrete aggregate particles, and the discrete element method (DEM) is the most widely adopted numerical method to simulate the particulate nature of ballast materials and their particle interactions. Large-scale triaxial tests performed in the laboratory under controlled monotonic and repeated loading conditions are commonly considered the best means to measure macroscopic mechanical properties of ballast materials, such as strength, modulus, and deformation characteristics, directly related to load-carrying and drainage functions of the ballast layer in the field. A DEM modeling approach is described for railroad ballast with realistic particle shapes developed from image analysis to simulate large-scale triaxial compression tests on a limestone ballast material. The ballast DEM model captures the strength behavior from both the traditional slow and the rapid shear loading rate types of monotonic triaxial compression tests. The results of the experimental study indicated that the shearing rate had insignificant influence on the results of the triaxial compression tests. The results also showed that the incremental displacement approach captured the measured shearing response, yet could save significant computational resources and time. This study shows that the DEM simulation approach combined with image analysis has the potential to be a quantitative tool to predict ballast performance.