By Evangelia Ieronymaki, Ph.D., P.E., Director and Senior Lecturer in the M.S. in Project Management Program, Columbia University School of Professional Studies
Dense cities face a fundamental infrastructure problem. Transportation requires space, but urban space is finite and increasingly valuable. For much of the 20th century, the solution was to build upward. Railways and highways were placed on viaducts above streets and neighborhoods. From an engineering perspective, these solutions were often practical and economical. From an urban perspective, however, their long-term consequences include noise, visual obstruction, physical separation of neighborhoods and permanent occupation of valuable surface space.
Underground construction offers an alternative. It allows transportation infrastructure and urban life to occupy essentially the same footprint at different elevations.
New York illustrates the scale of the issue. The MTA’s 2025-2029 Capital Plan identifies approximately 70 miles of elevated structures and viaducts within New York City Transit's roughly 250 miles of line structures. More than 20 miles of elevated structures are scheduled for repair during the current capital program. These structures remain throughout Brooklyn, Queens, the Bronx, and Manhattan, including the 1 train viaduct around West 125th Street.
These systems cannot simply be moved underground. Constructability, geology, utilities, foundations, service disruption, cost and asset condition must determine what is feasible. However, when major elevated infrastructure reaches the point of replacement, underground alternatives deserve serious consideration rather than automatically reconstructing infrastructure above the city.
Boston provides measurable evidence of what such a transformation can achieve. The Big Dig replaced the six-lane elevated Central Artery with an underground expressway containing 8 to 10 lanes. Following the project, total vehicle hours on project highways declined 62% between 1995 and 2003, generating approximately $168 million annually in estimated traveler savings. Citywide carbon monoxide levels also declined by 12%. The urban effects were equally important. Approximately 27 acres where the elevated highway had stood were reclaimed, with 75% designated to remain open. The broader project created more than 45 parks and major public plazas.
Seattle provides a second instructive example. The approximately 1.4-mile Alaskan Way Viaduct was replaced by the two-mile State Route 99 tunnel beneath downtown. The City of Seattle describes the former elevated highway as a physical barrier that generated noise, affected views, blocked sunlight, and separated downtown from the waterfront. Its removal enabled construction of the new waterfront environment, including pedestrian and bicycle infrastructure and Waterfront Park.
The Hudson Tunnel Project provides a particularly relevant contemporary example. Approximately 450 trains use the Northeast Corridor between Newark and New York Penn Station each day, accounting for 200,000 passenger trips. Today, trains crossing the Hudson depend on the two tracks of the North River Tunnel, which entered service in 1910.
The project is not adding tracks inside that existing tunnel. It is constructing an entirely new two-tube tunnel beneath the Hudson, containing two new tracks. Once it opens, the existing North River Tunnel will be rehabilitated. Ultimately, four modern tracks will be available between New York and New Jersey instead of two, providing substantially greater redundancy and operational flexibility.
This is important beyond the project itself. Additional transportation capacity across a river does not inherently have to take the form of another visible crossing. A bridge could theoretically provide an above-ground transportation corridor across the Hudson, although it was not the selected alternative for this project. Instead, a major new piece of infrastructure is being constructed beneath the river, preserving the waterfront and skyline while accommodating transportation below.
This principle should become increasingly important as dense cities consider future infrastructure: before adding another elevated railway, viaduct, or major above-ground transportation structure, engineers and planners should evaluate whether the required function can be accommodated underground.
The Hudson project also illustrates another benefit of infrastructure investment: the professional ecosystem required to deliver it. The Gateway Development Commission estimates that construction of the project will support approximately 95,000 jobs and generate approximately $20 billion in economic activity.
For project management professionals, the project provides a particularly useful case study. Gateway Development Commission and Amtrak positions assigned to the Hudson Tunnel Project show the breadth of project management functions required to deliver infrastructure of this scale. These include cost and schedule control, risk management, change control, construction management, systems integration, stakeholder coordination, testing and commissioning, and management of specialized systems such as tunnel ventilation and fire and life safety.
The environmental effect of underground transportation must be considered carefully. At the surface, benefits can include lower noise exposure, elimination of visual barriers, restoration of public space, new vegetation, improved pedestrian environments, and reconnection of neighborhoods. Boston's measured 12% reduction in citywide carbon monoxide provides one example of broader environmental improvement associated with a redesigned transportation network.
Tunneling, however, has its own environmental and engineering costs. Excavation is energy-intensive. Large quantities of concrete and steel carry substantial embodied carbon. Spoil must be transported and managed. Road tunnels require ventilation, lighting, drainage, fire protection, pumping, and continuous maintenance. Dense urban tunneling must also navigate utilities, foundations, groundwater, existing transportation infrastructure, and ongoing city operations. Boston's Big Dig, for example, excavated 16 million cubic yards of soil, requiring more than 500,000 truckloads to transport.
Underground infrastructure is therefore neither simple nor universally appropriate. It requires rigorous life cycle, cost, environmental, geotechnical, structural and operational assessment. But the underlying principle deserves greater consideration in dense cities. When major transportation infrastructure must be replaced or expanded, we should not assume that the surface is where it belongs.
The underground can move the city, while the surface should remain for the people who live on it.
Views and opinions expressed here are those of the author and do not necessarily reflect the official position of Columbia University School of Professional Studies or Columbia University.
About the Program
The Columbia University Master of Science in Project Management program equips individuals with the strategic, analytical, and leadership skills essential for a successful career managing complex projects across industries and borders.
Available full-time or part-time, the M.S. in Project Management is designed for professionals who want to advance into leadership roles or formalize their project management experience with a strong academic and practical foundation. Students can opt for the general Project Management program or choose from one of the four specialized concentrations: Construction, Sports Management, Sustainability Management, and Technology Management.
Taught by scholar-practitioners and enhanced by Columbia’s location in New York City, the curriculum integrates emerging digital tools and AI-driven practices to help graduates make data-informed decisions and improve operational efficiency. Graduates will be prepared to lead high-stakes projects with confidence and clarity, and return to the job market with a competitive edge.
The application deadline for the M.S. in Project Management program is June 1. Learn more about the program here.
