The Karnaphuli River Tunnel Project, Bangladesh’s first underwater tunnel, marks a major milestone in the country’s infrastructure development under the Belt and Road Initiative (ADB, 2020). This study examines the contrast between the project’s feasibility assessment and its real-world performance, focusing on technical, financial, and socio-economic aspects. The feasibility study predicted significant improvements in regional connectivity, trade efficiency, and national economic growth (China Communications Construction Company, 2018). However, the implementation phase faced challenges such as cost escalation, construction delays, and environmental impacts (Rahman & Ahmed, 2022). By comparing the projected outcomes with actual post-construction realities, this research evaluates the reliability of feasibility assessments in large-scale infrastructure projects in developing nations (World Bank, 2021). The findings underscore the importance of adaptive project management, realistic cost-benefit analysis, and continuous monitoring to minimize the planning–implementation gap (Hasan et al., 2023). Ultimately, the study provides insights into improving future infrastructure feasibility evaluations to enhance economic efficiency and sustainable development in Bangladesh.