In order to clarify and quantify the mechanism of the increase in the dynamic response of the bridge due to the passage of a train through a rail joint, this study conducted a numerical analysis of a conventional railway bridge, which is considered to be particularly affected by rail joints. The results showed that the dynamic response due to the passage of a rail joint is amplified by the resonance that occurs when the excitation frequency with the passage of two wheelsets in a bogie and the natural frequency of the bridge coincides, and is clarified the relationship between the resonance and the rail joint parameters. The impact factor at resonance was also found to be reduced due to the dynamic interaction between a train and a bridge. Furthermore, it is shown that the impact factor decreases with a decrease in the natural frequency of the bridge or an increase in the span, and that the impact factor is less than 0.05 for spans longer than 30 m.