Water content is one of the most important factors influencing the performance of earth structures. It is well known that the water content of sand has a significant effect on its shear strength and its resistance to cyclic loading. In this study, the loading resistance of four different sand materials was investigated in a cyclic load triaxial test at different water contents. Three of the four material samples were taken from the sub-ballast layer of an existing Finnish railway track. The laboratory tests confirmed that the resistance to cyclic loading decreased with an increasing saturation level for all tested materials. The weakest material in the cyclic loading triaxial test had the most uniform grain size distribution with a small median grain size. The calculation of the vertical stress increase in the sub-ballast layer revealed that even a fine-grained sand can withstand the train load well when the rolling stock is mostly passenger traffic with an axle load of 160 kN and the sub-ballast layer is not completely saturated. However, heavy freight traffic with axle loads of 225 kN and 250 kN can approach the bearing capacity of a weak sub-ballast material if the water content of the upper part of the sub-ballast layer is high. This study has shown that the top part of the sub-ballast layer should be kept below 60–70% saturation level to avoid the development of excessive permanent deformation.