Investigation of vibration is an important topic for the purposes of ride comfort in railway engineering. The vibration of rail vehicles becomes very complex because it is affected by the condition of vehicles, including suspensions and wheel profile, condition of track sections, including rail profile, rail irregularities, cant and curvature. This thesis aims at optimizing validated modelling methods to study its dynamics, how it influences on vertical vibration, how it excited on track and how it interacts with the passengers. The primary interest is ride comfort, considering the standard vibrations frequency range. In the standard frequency range, the structural flexibility of the carbody is concern. The models are intended for use in time-domain simulation, calling for small-sized models to reduce computational time and costs. Key parameters are proposed to select carbody eignmodes for inclusion in a flexible multibody model. Numerical simulations can be used to evaluate the comfort index at the railway vehicles. To this purpose, a melty-body modelling system is used that takes into account a string of factors influencing the level of the vertical vibrations, thus allowing a correct evaluation of the dynamic behavior in the railway vehicles, mainly at high velocities. The influence of the vertical suspension features upon the vibrating comfort will be examined, as a function of velocity. The concept of the critical point of the rail vehicle vibration behavior is introduced, as the point where the comfort index is the highest. While assuming the idea to minimize the level of vibrations in the critical point, it is thus proven the possibility of establishing the best damping of the vertical suspension that leads to the best values of the comfort index. Finally, the effects of different parameters and the optimized parameters are carried.