This paper discusses the importance of track irregularities in railway bridge design, and presents a new technique for calculating the dynamic impact load induced by such irregularities: the structural articulation method. The properties of the combined bridge-suspension system are coupled through global mass, stiffness, and damping matrices. Under the proposed method, the true suspension system over a particular point on the bridge girder at time t is divided into equivalent suspension systems attributed to adjacent finite-element nodes of the bridge. The time-dependent effects of a moving mass are thereby included in the equation of motion.
Lilley, D., Gu, G., & Franklin, F. (2010). A Structural articulation method for assessing railway bridges subject to dynamic impact loading from track irregularities. Vehicle System Dynamics, 48(10), 1077-1095. https://doi.org/10.1080/00423110903337273