A method to estimate the stress-free temperature in a rail by exciting the complete track has been theoretically investigated, by both simple beam theory and by a finite element model. For frequencies below a cut-off frequency, bending waves cannot propagate in the track. The cut-off frequency primarily depends on the stiffness of the lateral ballast. For a given ballast stiffness, the wavelength (or speed) of propagating waves depends on the axial stress in the rail. By first determining the cut-off frequency, the ballast stiffness can be determined. Then, the actual stress can be estimated by measuring the wave speed. By knowing the actual rail temperature, the stress-free temperature is then easily calculated. It is found that stress-free temperatures of 5 °Cshould cause measurable changes in the speed of wave propagation. It is determined that variations in damping of the ballast and stiffness of the pads in the rail clamps do not influence the results. Field measurements show that a track can be excited and propagating waves detected.