<p>Mud fouling progressively weakens ballast skeletons and may drive ballasted tracks from stable deformation to instability. Although geogrids are widely used to improve ballast performance, the effect of reinforcement layout under severe mud fouling remains unclear. This study proposes a pseudo-static finite-element stability assessment method based on plastic creep limit reduction to examine layout-dependent reinforcement effects in severely mud-fouled ballast. Cyclic triaxial tests were interpreted within the framework of shakedown theory to determine the plastic creep limits for sixteen combinations of fouling degree and mud-water content. The resulting critical stress states were expressed as linear instability boundaries in the p–q plane and introduced into the numerical model through field-variable-controlled updating of equivalent elastoplastic parameters. After identifying <i>VCI</i> = 40% and <i>MWC</i> = 58% as the baseline severe fouling condition, eight reinforcement layouts were analyzed, including unreinforced, single-layer, double-layer, and three-layer cases. The results show that geogrid reinforcement reduces stress concentration, restrains vertical displacement development, and improves relative stability. For single-layer reinforcement, the middle position gives the best overall effect, whereas the upper–middle layout performs best among double-layer cases. The three-layer layout provides the strongest stabilization, with no clear instability point within the explored reduction range.</p>