Subway Construction: Building a Unique Underground World

Most major cities in the world serve the public with large transportation systems, whether they’re above-ground light rails or trains bulleting through underground tunnels. As more people move to urban areas, subway systems will likely have to be updated — or even built outright in some places — which means they can be a lucrative area of business for a skilled general contractor (GC).

But subways are complex, sprawling, subterranean systems, and performing construction work on them can be a daunting task. In particular, the nature of subways gives them unique challenges in terms of cost, workforce, safety and ability to complete the job.

This article explores the process of subway construction work, the specialized and costly equipment necessary to do the job and the unique safety and jobsite considerations of working below street — and sometimes sea — level. 

Stages of subway construction

Working on an existing subway system — extending a track or building a new line, for example — can mean that the worksite is miles long and costs millions of dollars. Many aspects of subway construction mirror the phases of project management in other types of construction, but in general, the challenges at each step tend to be as large and complicated as the jobsites themselves. 

The following lists some of the challenges a GC could come up against in the different stages of construction. 

Planning and Design

Subway projects tend to be billion-dollar, drawn-out affairs. Since they’re typically funded by governments and through public-private partnerships, the process before even securing the bid can be years, even decades, long. 

For example, the most recent phase of the NYC Second Ave Subway project secured 3.4 billion from the Federal Transit Administration and began soliciting bids in 2023, but the Metropolitan Transit Authority (MTA), which operates the subway system, has been working on the master plan for that design since the early 2000s. 

Preconstruction

Getting ready to start the build requires obtaining permits from relevant authorities having jurisdictions (AHJs), hiring a competent workforce, creating safety and material delivery plans and meeting with stakeholders.

However, the sheer number of stakeholders involved in a subway project can be overwhelming. In addition to regulation from the Department of Transportation, often the local operating authority — the MTA in New York City, for example — has a heavy hand not just in preconstruction meetings but also in their own approval processes. 

They may employ their own environmental engineers, project managers and specialists to work with the contractor’s own team and approve the details of each preconstruction decision. 

This structure, although designed to maintain quality and oversight on projects using taxpayer funds, can feel slow and unwieldy to a contractor not used to getting so much external approval.

Since the worksite is so big, the GC should plan to hire a large workforce. They’ll need standard tradespersons — like electricians, mechanics and plumbers — but they’ll also need to work with the local tunneling and dirt excavation trade, like the Sandhogs in New York City. It’s also a good idea to work with local fire rescue teams to generate an evacuation plan in case of emergency, and to train crewmembers on the procedure regularly.

Finally, since subways tend to be under streets, the GC must work with the city to determine what lanes or streets will need to be closed, and for how long. 

Construction

Before even starting on the actual subway tunnel, the GC and crew must build a launch box. The launch box is a designated area used for assembling and launching specialized equipment — in this case, the tunnel-boring machine (TBM). It also includes facilities for maintenance, storage of materials, and logistical support, and will remain after the build is complete. 

To make a launch box, workers blast and drill into the street and lower it into the hole. An important part of this process is utility relocation. Water mains, electrical lines, phone cables and gas pipes may all need to be re-routed during this phase.

Once the relocations are finished, actual tunnel excavation begins, starting from the launch box. The TBM is assembled and deployed, carving out the tunnel according to the project's specifications. 

While building a tunnel requires a lot of earthwork, a subway system is a unique feat of electrical and mechanical engineering. Other aspects of construction include laying tracks, installing switches, signals, cars and rails — including the third rail, the live conductor that moves the train. 

Above ground, ventilation buildings will have to be installed at regular intervals for both the safety of future passengers and crewmembers. 

Materials — concrete, bars for both the tunnels and tracks, for example — are likely to arrive as just-in-time deliveries, and because of the huge amounts needed, a GC may need to source from several companies. These materials — as well as any other equipment — will need to be dropped down into the tunnel with a crane.

Managing waste during the construction phase can also present challenges. A system will need to be rigged to lift debris out of the tunnels as needed, and be sorted into a dumpster nearby. Since subways tend to be in large metropolitan cities, space for waste could be limited, and the nearest landfill could be far away. 

Project Closeout

When subway construction is near complete, it enters the commissioning phase, during which the transit authority conducts the final checks and approvals. This phase also involves meticulous inspections to ensure that all signals, switches and installations are functioning correctly. Finally, it will include test runs with rail cars to ensure that the system is ready for public use.

Typically, the opening of the subway will involve an inaugural train ride, which signifies the end of years-long planning and construction efforts. 

Additional Challenges of Subway Construction

It’s hard to overstate the safety risks of asking a crew of hundreds to tunnel underground. Site superintendents should always know how many people are inside the tunnel at any given time, which may require additional documentation and logging processes that some crewmembers aren’t used to.

Jobsite security

In addition to safety, security measures might be needed to protect the integrity of the tunnels once they are built. Security cameras might be useful at every tunnel access point, and security officers may need to be stationed. Measures like these aren’t just about keeping out potential trespassers; they’re about ensuring that every worker knows they’re operating in a safe, secure environment. 

Working underground

Working underground also requires air and dust monitoring, and may require extra PPE like respirators to keep workers from being exposed to harmful pollutants. 

But some subways don’t just run underground — they also run underwater. Dewatering pumps operate constantly in areas with high groundwater levels or where tunnels are below the water table. These pumps are designed to remove excess water from the excavation site, keeping the work area dry, and they will remain in place after construction ends to maintain the system. 

The Bottom Line on Subway Construction

There’s a reason that the New York Subway System began construction in 1900 and wasn’t truly close to the bustling apparatus we know today until the F line opened in 1989: Subways are long projects, built and renovated in many stages — and are never truly finished.

Since they’re ever-growing and ever-needed, subways are also an incredible source of profit for the adept contractor. Though building a subway line is a challenging feat of literal earth-shaking proportions, this work is also necessary to the function of society, and a monumental gift to a city’s current and future residents. 

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