Abstract: An interlayer soil was identified in French conventional rail tracks, corresponding to a mixture of subgrade fine soil and ballast grains. To investigate the role of fine soil on the resilient modulus M_r of the mixture, three water contents of fine soil w_f (17.6%, 13.7% and 10.6%), five coarse grain contents f_v (0%, 10%, 20%, 35% and 45%) and five deviator stress amplitudes σ_d (10, 15, 20, 25, 30 kPa) were considered. Multi-stage cyclic triaxial tests were conducted for the determination of M_r. Mercury intrusion porosimetry tests were performed for the microstructural observation of the fine soil. A uni-modal porosity microstructure, characterized by a global fine matrix, was identified at w_f =17.6% and 13.7% (> plastic limit of fine soil w_p=12%), while a bi-modal porosity microstructure with well-developed aggregates was identified at w_f =10.6% (< w_p=12%). Interestingly, combined effects of w_f and σ_d on M_r of the mixture were found. Increasing σ_d led to a decrease of M_r when w_f>w_p, but an increase of M_r when w_f<w_p. This led to the conclusion that, for the fine matrix fabric (w_f>w_p), an increase of σ_d led to a decrease of M_r, while for the fine aggregate fabric (w_f<w_p) an increase of σ_d led to an increase of M_r. The distinct M_r - σ_d behaviors for these two fabrics were attributed to the competing effects between soil hardening during the loading procedure and rebounding during the unloading procedure. For the fine matrix fabric (w_f>w_p), due to its high deformability, the rebounding effect on M_r appeared to be more significant than the hardening effect, resulting in a decrease of M_r with increasing σ_d. By contrast, for the fine aggregate fabric (w_f<w_p), owing to its low deformability, the rebounding effect on M_r was not as significant as the hardening effect, and thus an increase of M_r with increasing σ_d.