A statistical study was firstly undertaken on problems that occurred in a period of more than 1 year and were related to train circulation in the entire conventional French railway network. Emphasis was put on the degradation of track components. The analysis evidenced the particular importance of sub-grade quality in the performance of the whole track. After this general analysis, a conventional railway line in the West of France was investigated. In the sub-structure of this line, an interlayer was identified that was created mainly by interpenetration of ballast and sub-grade. In the analysis, the degradation rate of this line was correlated with different parameters, such as the nature of sub-grades involved along the line and the thickness of different layers. The results showed that the degradation rate is correlated to the thickness of various layers. Furthermore, it was observed that the interlayer plays an important role in the performance of tracks, as it represents a transition layer for the train-induced stress transmission to the sub-grade.