Europe's Rail Joint Undertaking (ERJU) is advancing the Single European Rail Network and modernizing rail transport systems. A key initiative is demonstrating a novel train protection system using moving block principles, which requires high safety assurance. STPA was chosen for hazard analysis due to its early design applicability. This presentation discusses the integration of STPA within ERJU, focusing on its role in the Moving Block ETCS Level 3 Specification project. It addresses challenges in providing feedback for requirements development, validating assumptions with experts, and ensuring the analysis impacts various work packages.