On the thin-walled steel members of railway bridges, it has been clarified by measuring the vibration of a railway bridge actually that local vibration of the members increases suddenly with speeding up of train. This paper aims to elucidate the mechanism of the sudden increase of the local vibration. Firstly, the phase velocity and group velocity of the local vibration in steel component plates of thin-walled steel members are computed. Secondly, the dynamic responses of the steel component plates under high speed moving loads are analyzed. The relation the local vibration with the speeding up of the moving loads are investigated on wave form, time histories, reflection and propagation of local vibration of the plates.