In French conventional railway track-bed, an interlayer was created, mainly from the interpenetration of ballast and subgrade. Field investigation showed that the ballast content decreases over depth. In addition, depending on the locations, the grain size distribution of ballast grains can be quite different, defining different values of coefficient of uniformity, Cu. In this study, triaxial tests were carried out on the interlayer soil with three Cu values and five volumetric inclusion contents, fv. X-ray micro computed tomography scans were also conducted on representative samples to visualise the corresponding inclusion distributions. Results show that: (a) at all three Cu values, two soil fabrics can be identified, namely grain–grain contact structure for large inclusion contents (fv = 35% and 45%) and fine–fine contact structure for small inclusion contents (fv = 5%, 10% and 20%), and the characteristic volumetric inclusion content fv-cha separating these two categories was larger for smaller Cu since in this case fewer inclusion contacts were expected to be developed at a given fv; (b) for the grain–grain contact structure, a smaller Cu caused larger maximum deviator stress qmax, larger friction angle, larger Poisson ratio and more dilatancy due to the involvement of more large grains at smaller Cu; (c) for the fine–fine contact structure, an opposite trend was observed: the smaller the Cu, the smaller the qmax, the friction angle, the Poisson ratio and the dilatancy owing to a smaller quantity of inclusion grains; (d) the change patterns of Young's modulus and cohesion with increasing Cu were the same for the two soil fabrics – Young's modulus and cohesion decreased with the increase of Cu.