Evaluating environmental sustainability and climate change impacts, particularly concerning the lifespan of high-speed railway (HSR) infrastructure, poses notable challenges for traditional cost–benefit analysis (CBA). Recent methodologies provide guidance on adapting CBA to address these complex issues more effectively. A briefing from the European Environment Agency (EEA) highlights the importance of a thorough understanding of the trade-offs in climate adaptation strategies, as the European Union faces increasing financial impacts from extreme weather events. This study identifies gaps in conventional CBA guidelines for HSR, suggesting alternative discount rate scenarios for decision-makers that incorporate long-term perspectives. Approaches such as variable discount rates for environmental impacts and explicit consideration of intergenerational effects offer ways to refine CBA for infrastructure projects. By re-examining current CBAs, this research illustrates how the analysis can evolve to meet the growing demands of sustainable, resilient infrastructure planning.