Abstract: The railway network in the Loess Plateau routinely traverses high-density farmland irrigation corridors, where the subgrade continuously accumulates excessive post-construction settlement, severely threatening the millimeter-level smoothness control requirements of railways and posing significant challenges to train operation safety. This study addresses the prominent conflict between routine farmland irrigation practices and the stability requirements of railway subgrades in this region. Through field investigations at a typical site along the Baoji-Zhongwei Railway and dynamic triaxial laboratory tests, the causes of subsidence in loess subgrades under irrigation were identified, and the dynamic residual deformation characteristics of loess after wetting were revealed, along with proposed prevention-control thresholds. The research demonstrates that periodic irrigation-induced moisture fluctuations in the subgrade soil, combined with long-term cyclic train loads, are the primary drivers of severe settlement. By conducting comparative simulation tests of train load waveforms under varying drainage conditions, it was found that the residual strain under semi-sinusoidal wave loads decreased by 15%~20% compared to conventional sinusoidal waves, confirming that semi-sinusoidal waves better reflect real-world train dynamic load characteristics. A dynamic residual strain model for loess was established, incorporating consolidation stress, dynamic stress amplitude, and vibration cycles. Preliminary predictions of subsidence in actual irrigation zones using this model aligned well with field observations. The findings provide a theoretical foundation for long-term settlement prediction of railway subgrades in loess regions and hold significant engineering value for advancing lifecycle safety assessments of subgrade defects under coupled vibration-seepage interactions.