• Wave barrier design for mitigation of underground train vibrations. • Considering layered grounds with different groundwater table levels. • Wave propagation analysis in saturated grounds using Biot’s theory. • Exploring the effects of loading frequency and barrier geometry. • Using a coupled CMA-ES/FE procedure for the optimal barrier design. This paper explores using wave barriers for mitigating vibrations generated by underground railway transit. An optimal design procedure is developed by coupling finite element (FE) numerical simulations and covariance matrix adaptation evolution strategy (CMA-ES) to find the optimal barriers’ configurations in different situations. The optimization variables include width, height, inclination angle, and barrier location. Various scenarios are considered to study the real problems encountered in practice, including the relative position of the tunnel and the protecting structure, ground layering, and groundwater table (GWT) level. Furthermore, the effect of the frequency content of the underground train loading is accounted for due to its significant importance. The soil beneath the GWT level is considered fully saturated, and Biot’s poroelastodynamic theory is used for wave propagation analysis. It is observed that the optimal wave barriers generally tend to locate in an active situation, close to the loading source. The ground layering, GWT level, and loading frequency significantly affect the barriers’ characteristics and the amount of vibration mitigation. The results show that the optimal barriers generally have large geometric aspect ratios. Also, barriers have more influence in the homogenous grounds because the wave scattering at the interfaces in the layered ground mitigates the waves naturally.