The evolution of the European Rail Traffic Management System (ERTMS) to level 3 enables increasing the capacity on the lines thanks to continuous supervision and communication, moving block and virtual block. It relies on the autonomous position and integrity information reported by the train to determine if it is safe to issue to 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.