A top bridge research priority for railroad owners is to determine bridge displacements under in-service train loadings. Recent studies have demonstrated that displacement data assists railroad owners in prioritizing maintenance, repair, and replacement policies. However, displacement measurements are expensive to collect because of the need for static reference points. In this research project, UIUC has developed a straightforward and cost-effective means to estimate reference-free bridge displacements under in-service train loads from measured accelerations. The proposed system was validated for railroad bridges of higher interest/need of condition assessment by Class I railroads, such as timber trestles and steel trusses, with an emphasis in pin-connected trusses.