Previous studies showed that the permanent strain of interlayer soil in the French conventional railway substructure is highly dependent on the volumetric coarse grain content fv (volumetric ratio of coarse grain to total sample). This study developed a fatigue model allowing the effects of coarse grain content fv, stress state and number of loading cycles N on permanent strain to be accounted for. Data from the multi-stage loading cyclic tests of interlayer soil conducted at six different fv values (0%, 5%, 10%, 20%, 35% and 45%) and five different maximum deviator stress ?qmax values (10 kPa, 15 kPa, 20 kPa, 25 kPa and 30 kPa) were reviewed and used for this purpose. The model parameters were determined by fitting the results from the tests at fv = 0%, 5% and 35%. In order to validate the proposed fatigue model, the determined model parameters were then used to simulate the tests at fv = 10%, 20% and 45%. The results obtained showed that the proposed model can well describe the permanent strain after a certain number of loading cycles, in the case of plastic shakedown.