In estimating the risk of flange-climb derailment, it is generally thought that a wheel will not derail if the derailment coefficient is smaller than the critical value calculated with "Nadal's equation." To estimate derailment coefficient, time-series simulation is often performed, but this needs a long calculation time. As a quicker method, we propose to estimate the derailment coefficient from analytically calculated lateral and vertical wheel loads, based on analytic equations and measured data. This paper describes the equations used, shows how the derailment coefficient is calculated, and compares it with the results achieved by measurements.