The evolution of the European Rail Traffic Management System (ERTMS) to level 3 enables increasing the capacity on the lines thanks to continuous on-board train integrity supervision and communication, moving block and virtual block. It relies on the autonomous position and integrity information reported by the train to generate a movement authority. The aim of our paper is to present a short preview of the train integrity monitoring system and its safety challenges. A methodology to maintain traceability of safety requirements to cover the hazards, identified from the system functions, is proposed based on the standard EN 50126.