The main stiffness properties that determine the stresses in the track structure components under loads from a moving train are the modulus of elasticity of the rail slab and the slab-track/rail correlation stiffness coefficient. These parameters have been investigated for a ballasted track and are well established today, in contrast to those of a non-ballasted track. This study aims at determining the stiffness characteristics of a non-ballasted track, comparing them with those of a ballasted track, and assessing their effect on the stress-strain state (SSS) of the elements of a non-ballasted track structure. Field experiments to measure the stresses in the track structure elements were carried out using strain-gauge methods. As a result of the experiments, the modulus of elasticity and the correlation stiffness coefficient of the rail slab and the rail were determined for the RHEDA2000 slab track system. The results obtained prove it possible to apply a rail-as-beamon-elastic-foundation theory and to use well-established calculation methods for designing a non-ballasted track structure suitable for different operational conditions.