Since differential displacement of track surfaces caused by earthquakes strongly influences the running safety of trains, it is important to consider its effects for the seismic design of railway structures. In this paper, in order to obtain a proper seismic wavelength for the assessment of track differential displacement, the authors examined phase velocity characteristics which reflect the dispersion of Rayleigh waves, and proposed an empirical formula for calculating the wavelength suitable for track assessment based on sets of various real ground parameters. Moreover, the influence due to the wavelength and the characteristics of the ground on the angular rotation assessment of viaducts was examined.