This paper presents the research framework and preliminary findings from a recently started Federal Railroad Administration (FRA) sponsored research project at the University of Illinois at Urbana-Champaign on developing and using seismic wave propagation principles to develop a track substructure (ballast and subgrade) condition assessment system. This condition assessment system will be used to evaluate track safety and to predict inspection intervals. This research is needed because there is no tool or system available that quantitatively measures the mechanics based properties of the layers comprising a track system. These mechanics based properties are shear modulus, modulus of deformation (Young's modulus), and shear strength that can be derived from direct measurement of the shear wave velocity profile. Currently the modulus and shear strength of the track system materials are usually estimated which leads to uncertainty in the numerical analyses and estimates of track safety.