Like most transport-related megaprojects, decisions for new High-speed Railways (HSR) face criticism and raise questions about their effectiveness. HSR projects frequently encounter construction cost overruns and overestimations of travel demand. France is no exception, although its geographic and social characteristics favor HSR expansion. To analyze the specific conditions in France, this study examines key indicators related to construction costs and user impacts across the French HSR network, with additional consideration of demand implications. The analysis is structured into three phases: preliminary (project proposal), initial (start of construction), and final (operational phase). Findings reveal an average increase in construction costs of 13% from the preliminary to initial phase, followed by a further 5% increase from the initial to final phase. These cost overruns are primarily due to modifications in project plans, such as the addition of stations. Cost increases are more pronounced in recent lines, while older lines (e.g., Paris-Lyon, Paris-Lille) show minimal increases from initial estimates. The study also compares France with Italy, highlighting similarities and differences in design and construction efficiency. The findings provide valuable insights into the expertise and project management capacity of HSR authorities in both countries. In Italy, persistent cost overruns reflect issues in cost forecasting rather than unforeseen events, suggesting that a more rigorous preliminary assessment could lead to more accurate forecasting and smoother project delivery. The French example demonstrates that such an approach is feasible and may reduce stakeholder conflicts.