The interface damage as one of the most critical damage issues in railway slab tracks is evaluated in this work on the basis of reliability techniques and vehicle-track interactions. First, a coupled dynamics model of a vehicle and the slab track is developed involving nonlinear spring-damper elements for simulation of the interface damage. Furthermore, considering the random nature of the damage length, the damage height, the rail pad stiffness, and the elastic modulus of cement asphalt (CA) mortar layer, explicit mathematical expressions between the input stochastic variables and output dynamic responses are obtained on the basis of the combination of the response surface method (RSM) and the dynamic simulations of vehicle-track system. Subsequently, Monte Carlo (MC) simulations are performed for the probability analysis by directly using the response surface functions. Finally, by adopting the amplification factor (AF) of the dynamic response as the control indices, the damage assessment criterion and the corresponding safety threshold are suggested on the basis of the concept of reliability for the long-term dynamic performance of slab tracks. The results show that the interface damage has a significant influence on the slab displacement, the slab acceleration, and the dynamic stress of CA mortar layer; the damage length is the primary control index for the interface damage with a complete failure along the width of the slab track; the interface damage can be evaluated as damage levels I, II, and III when the survival probabilities for the AF of exceeding a certain value are 50, 30, and 10%, respectively; the safety thresholds for the interface damage levels I, II, and III are suggested to be 0.60, 0.85, and 1.05 m, respectively.