<p>Long-term deformation of heavy-haul railway subgrades under repeated traffic loading is a critical factor governing track performance and maintenance demand. To investigate the deformation behaviors and shakedown characteristics of compacted silty clay subgrade soils, a series of cyclic triaxial tests were conducted under different cyclic stress ratios, moisture contents, and compaction degrees. The evolution of irreversible deformation with loading cycles was systematically analyzed, and distinct response regimes corresponding to plastic stability, plastic creep, and incremental failure were identified. An empirical model was adopted to describe the development of permanent deformation with cycle number, and the model parameters were quantitatively related to key state variables. The results show that higher cyclic stress ratios and moisture contents significantly accelerate deformation development, whereas increased compaction enhances resistance to deformation accumulation and promotes stable behaviors. Based on the proposed model, shakedown thresholds were determined, providing a practical framework for evaluating long-term subgrade performance under heavy-haul traffic. The findings offer insight into the combined effects of stress level, moisture condition, and compaction on subgrade deformation, and may support the performance-based design and assessment of heavy-haul railway subgrades.</p>