Track sub-structural stiffness is a major factor in the deterioration of track geometric quality as a result of the rail traffic that passes over it. Whilst track geometric quality has been measured at normal line speed for many years, determination of the condition of the substructure has only been possible at discrete locations using stationary devices or invasive manual inspection. This paper describes the problems caused by differential track system stiffness and progress with research that seeks to identify and implement a suitable technology that will enable the measurement of track structural stiffness at normal linespeed. Acquisition of this information on a production basis and use of it in conjunction with other condition data collected routinely offers Network Rail the opportunity to progress towards more effective targeting of track maintenance and renewal resources through greater knowledge of whole track system condition. (6 pages)