At Mid Niigata Earthquake in October 2004, a Japanese high speed train was derailed under commercial operation at the speed of 200km/h. Investigations of the case concluded that the horizontal ground motion by the earthquake was the major cause of the derailment, which implies that railway vehicles could be derailed solely by the ground motions of earthquakes with no fatal damages of vehicles or tracks. Based on the reports and facts, we believe that we should study both the derailment mechanism and function of guard rail, to pursue to minimize the risk of railway system against large earthquakes. In this paper, we developed a simulation program to study the basic mechanism of the function of guard rail to prevent derailment due to large eathquakes. Then through numerical analysis based on the simulation program, we obtained the following results. Firstly, anti-derailing guard rail is effective to prevent derailment due to large track excitations. Secondly vertical ground motions, light weight body and curve running on canted track have relatively smaller influence on the derailment mechanism.