A real-time prognostic tool to predict life-time of IGBT power modules in a metro application is presented. Applying conventional life models (e.g. Coffin-Manson) for real applications is infeasible because these models are only applicable to cyclic data. Use of off-line rainflow algorithm is common solution but cannot be applied in real-time in its original form. This paper presents on-line life-estimation of the power modules using real-time rainflow coding algorithm. This technique is applied to an example metro application that requires use of cycle counting for an arbitrary load profile. The proposed method uses a stack-based implementation which employs a recursive algorithm to identify full and half cycles of the temperatures obtained as outputs from real-time compact thermal models. This then allows life-time models to be used to provide life consumption estimates. This method provides less complexity and more accurate on-line prediction for the studied module's failure mechanisms.