The purpose of this research is to improve the running safety and riding comfort of urban rail transit. A typical continuous box bridge in the Beijing Metro line 5 is selected. A coupling vibration model of bridge-vehicle-passengers is established， and the differential equations of motion are deduced， then a corresponding calculating program is written. The dynamic deflections of the bridge， train， and passengers are calculated exactly. The changing safety of the maximum values of each responses， and the running stability and safety indexes of the train are analyzed， too. The reliability of the simulation model and calculating program are verified by the monitored data. The results proved that the selected bridge is in pretty well working conditions. The vibrating acceleration， lateral forces， offload factor， and derail factor of train are quite perfect at current running speed. There are obvious differences between passengers' vibration responses and the vehicles'， and there is a little hysteresis. Dynamic responses of passengers sitting in the middle of the carriage are a little smaller than that in both end， and those sitting in the middle carriage are smaller than in the end carriages.