Excessive uneven settlements of railway subgrades will reduce the durability of ballastless slab track and jeopardize the train operation safety. In this paper, a technology by injecting polyurethane materials beneath the slab track to uniformly lift the track level is introduced to eliminate track uneven settlement and to ensure its good performance. The mechanical properties and long-term durability of track/roadbed systems that have been rehabilitated by polyurethane grouting are presented. The cyclic uniaxial loading test shows that the polyurethane material has good stability and durability after 3,400,000 cycles of loads; full-scale ballastless track model test shows that the stiffness of track-subgrade system initially decreases by 20% after grouting, resulting in an increase in the track dynamic responses, and after 790,000 cycles of train loads, the stiffness of the grouted subgrade gradually increases while the accumulative settlement is less than 3 mm for maximum 46.12 mm uplift, indicating that the polyurethane-grouted subgrade has good dynamic stability under long-term train loads. Afterwards, this technology was successfully applied in restoring track geometries in an uneven settlement zone of a high-speed railway line on the east coast of China. In-situ tests show that the polyurethane grouting significantly improves the track geometries and control the post settlements after grouting. According to the lab and in-situ tests, it is demonstrated that the polyurethane grouting technology can effectively restore ballastless track uneven settlements and provides adequate long-term durability under moving train loads.