The condition of the underlying subgrade in the slab-track system plays a critical role not only in the performance of the slab-track but also significantly affects the internal force response of the system. This paper investigates the effect of non-uniform subgrade conditions along the slab-track system's cross-section in longitudinal and transversal directions. The non-uniformity of the subgrade condition was modeled by varying the spring stiffness at the localized area located at the middle cross-section of the slab track system. The effect was evaluated by comparing the ideal and degraded models’ bending moment diagram (BMD) and the shear force diagram (SFD). The analysis found that the subgrade modulus's non-uniformity affects the slab track's BMD and SFD. The lower the subgrade modulus stiffness in the defect area, the more pronounced the BMD and SFD changes. This finding signifies that during the design, the expectation of the possibility of having a non-uniform subgrade modulus should prevent premature failure in the slab-track system.