What are the primary considerations and challenges associated with the design and implementation of large$-$scale underground transportation systems in urban environments? Analyze the complex interactions between geotechnical conditions, structural integrity, and environmental impacts within the context of tunneling and excavation. Explore the multifaceted aspects of material selection, such as concrete, steel, and composite materials, in ensuring the durability and safety of underground structures. Investigate the role of advanced technologies, including tunnel boring machines $($TBMs$),$ in optimizing construction efficiency and minimizing disruption to existing infrastructure and communities. Assess the implications of groundwater management and soil stabilization techniques on the long$-$term stability and performance of underground transportation networks. Delve into the socio$-$economic factors influencing project feasibility and stakeholder engagement, addressing issues related to cost$-$effectiveness, land use planning, and public perception.