With the recent dramatic increase in the daily number of vehicles running on train lines, considering the effects of simultaneous train loading on double-track bridges has become a significant factor in fatigue design for railway steel bridges. In this research, we assessed the effects of simultaneous loading considering several parameters by applying Monte Carlo simulation, and evaluated each parameter's degree of contribution to fatigue damage in simultaneous loading. Using the results obtained, we proposed a new method for practical design calculation that can easily be adopted to consider simultaneous loading on double-track bridges using a coefficient.