<p>The traditional steel spring floating-slab track (FST) is a linear vibration isolation system, which cannot effectively isolate low-frequency vibrations. Although the quasi-zero stiffness system can effectively suppress low-frequency vibrations, it is suitable for vibration isolation at the equilibrium position. Once the system deviates from the equilibrium position, its vibration isolation performance decreases. To address the above problems, this paper innovatively designs a Halbach-array torsional nonlinear vibration isolator (HTNVI) based on the original design dimensions of the steel spring isolator. By using the Halbach magnet array combined with a ball screw system to form a negative stiffness structure, the issue of the negative stiffness restoring force generated by the negative stiffness structure. When combined with the linear restoring force, HTNVI can create multiple low-stiffness bearing regions. These regions, within the displacement limits of the FST system, enable load adaptability. Theoretical analysis shows that the HTNVI exhibits good vibration isolation performance under low-frequency excitation and load variation conditions. Subsequently, a vehicle-FST-isolator finite element simulation platform was established. Vibration isolation performance of the HTNVI on the FST was analyzed. The simulation results show that compared to the steel spring isolator FST system, the displacement response of the FST system using the HTNVI is reduced by about 50%, the acceleration response is reduced by about 60%, and the support reaction force is reduced by about 16%. Therefore, this research provides new insights for low-frequency vibration isolation design in FST systems.</p>