In simulation of evaluating a park-and-ride system, a timetable of associated public transportation strongly affects the simulation results. Conventional traffic-flow simulators that examine the effectiveness of a park-and-ride system are insufficient, because it is not practically possible to consider all timetables of the public transportation. In this study, using a mathematical optimization technique, the authors develop a method to obtain an optimal train timetable for a road network employing a park-and-ride system. The proposed method enables the authors to assess the effectiveness of a park-and-ride system more accurately to reduce congestion on a road network.