The contact forces between the wheel and the railhead control the dynamics and fatigue life of both the wagon and the track structures. Further to these low frequency contact forces, rail joints and defects in railheads and wheel surfaces generate high frequency contact-impact forces. Only expensive systems such as the fully instrumented wheelsets can measure these forces accurately. This paper deals with an inexpensive wayside monitoring technique that determines the contact-impact forces inversely through strain signatures. Insulated rail joints (IRJ), that are safety critical components of the railway signaling system are particularly referred to in the wayside experiments. IRJs are characterised by short service life and hence of interest to the rail industry. The method presented in the paper could be equally well applied to measuring contact impact forces due to any other type of joints or railhead defects.