<p>The time-dependent reliability assessment of ballastless tracks in service life is crucial for ensuring the safe and stability of high-speed railways. This work systematically compiles and summarizes the performance functions of ballastless track structures under different failure modes during service period, and then the all modes are regarded as a mixed series-parallel system, a regularization method for performance functions of multiple failure modes is proposed. Subsequently, the direct probability integral method (DPIM) is firstly introduced for evaluating the time-dependent reliability of the CRTS III slab ballastless track system. The accuracy and computational efficiency of the proposed method are verified through comparison with the traditional Monte Carlo method. Furthermore, the failure modes of the system are categorized into safety and applicability. Numerical examples demonstrate the generality and robustness of the developed method in system reliability analysis. The result indicates that environmental factors are the primary cause of diminished system reliability, both in terms of safety and applicability. Extreme temperature gradients are identified as a primary cause of safety performance degradation in track structures, while negative temperature gradients primarily contribute to serviceability deterioration. The mean of train load below 300 kN is essential for maintaining safety performance. The safety and applicability of the track structure after 30 years of service should be a key concern for the railway department. Especially, the reliability calculation method developed in this work for CRTS III systems can be extended to various static and dynamic system state evaluations.</p>