An on-board traction transformer with online monitoring systems for predictive and diagnosis maintenance is presented. Today, no tele diagnosis function is connected with this component on rolling stock. Maintenance operations are made according to the suppliers' advice and return of experience. In power transformer area, the monitoring on the transformer and its accessories is essential, due to criticality for the exploitation in case of major failures. Online diagnosis to simplify the maintenance and to improve the availability of the transformer and also on the traction drive are studied.