Particle shape plays a key role in the mechanical performance of coarse-grained materials in civil engineering applications, particularly when particle breakage occurs. This study experimentally investigates the effect of particle breakage on shape factor changes of angular railway ballast particles with pyramidal and cubic geometries in the ballast size range of 2–6 cm. To minimize the inherent variability of natural aggregates, artificial particles were fabricated from reactive powder concrete (RPC), with compressive strength comparable to that of natural ballast. Pyramidal and cubic particles were then subjected to compressive loading until failure, and their geometric changes were quantified using the relative shape factor. The shape factor distributions were classified into four categories: low, medium, high, and very high. The results show that the initial particle geometry strongly affects both breakage behavior and the post-loading distribution of shape factors. For pyramidal particles, about 84% fell within the medium range, and the remaining particles were classified as low. In contrast, cubic particles exhibited a wider distribution: 25% were in the high and very high ranges, 69% in the medium range, and only 6.5% in the low range. These findings indicate that the initial particle geometry is a governing factor in the extent to which the shape factor is retained or reduced after breakage. PSD analysis showed a higher Hardin breakage factor for pyramidal particles (20.77%) than for cubic particles (12.91%), despite cubic particles having a greater reduction in relative shape factor, suggesting distinct mechanisms govern breakage extent and shape change. Furthermore, pyramidal particles showed less dispersion among the defined shape factor ranges than cubic particles, indicating greater geometric stability of pyramidal forms under compressive loading.