Flange climb derailment of railway vehicles occurs during low-speed running on sharply curved and twisted tracks. We studied the mechanism behind flange climbing to clarify and quantify the factors that cause derailment. In this study, passenger vehicle running tests were carried out on an RTRI test line subsidized by the Ministry of Land, Infrastructure and Transport. Derailment actually occurred during the tests, which measured wheel/rail contact forces, the forces exerted on the wheel sets and the dynamic behavior of the vehicle. Bench tests were also performed using roller rigs to investigate the characteristics of creep force. Based on the test results, this paper proposes a method to assess levels of safety against vehicle derailment on curved tracks.