Criteria for evaluating safety of railway vehicles concerning derailment are reviewed. In the case of quasi-static wheel climb derailment, the safety criteria in current use are available. Recently oscillatory wheel load fluctuations of considerable amplitude have been observed on Shinkansen vehicles running at high speeds, but there were no established evaluation methods for the dynamic derailment under such a specific condition. Based on a study on these dynamic phenomena through computer simulation, we propose to use the time duration of Y/Q in order to assess the safety against derailment. Moreover, the necessity to control the right to left static wheel load unbalance ratio and its limit value on assembling bogies are suggested in this paper with field data obtained from turnout running tests.