The main target of RAIL-EQ is to develop a holistic methodology, aiming to overcome drawbacks considering dynamic analysis of mechanical systems including moving mass and unilateral and bilateral constraints; in particular drawbacks related to mathematical description of the problem that may result in constraint violation, loss of stability and finally convergence and accuracy problems. In line with the main target which is the assessment of seismic behavior of existing railway bridges considering bridge and high-speed train interaction in the frame of a global railway network assessment and upgrade, component damage states are defined and a new methodology for bridge-specific analysis is proposed, dully tailored to be applied to a railway bridge inventory. Advanced co-simulation techniques are used for the coupled analysis of train and railway bridge, accounting for moving mass and unilateral and bilateral constraints. The later constitutes a novel proposal, applied using the software developed. It should be outlined that application of the mathematical formulation described above has already been applied on mechanical systems (vehicles, robotics) rendering the application on a complex system rather appealing and innovative.