Abstract: Dynamic responses of the rail structure and underground railway tunnel in the saturated soil under moving harmonic load are investigated by using the analytical method. The tunnel is simulated as a thin cylindrical shell with infinite length, and the soil is modeled as a saturated poroelastic medium using the Biot’s theory. The infinite track with periodic double beam units is formulated as a periodic structure. The tracks and soil medium are coupled by the force and displacement compatibility conditions at the tunnel invert. The effects of load velocity and external frequency on track responses and displacements and pore pressure of saturated soil are investigated. The dynamic characteristics of floating slab tracks with discontinuous and continuous slabs are analyzed. It is found that there exists a parametric excitation in the spectra of track and soil responses due to the moving load periodically passing through the discontinuous slabs. Resonance phenomenon occurs when the external frequency of load is equal to the natural frequency of the tracks. The dynamic characteristics of floating slab tracks with discontinuous and continuous slabs have significant difference. The displacements and pore pressures of saturated soil will be amplified at the natural frequency arising from the standing waves propagating in the discontinuous slabs. The displacements of the saturated soil can be effectively reduced by increasing the tunnel thickness.