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6 of the Longest Tunnels in the U.S.

By Bradley O'Neill
Read time: 6 minutes

According to a 2023 report by the Federal Highway Administration, there are 562 road tunnels spread across the United States. But road traffic isn’t the only use for these complex underground structures that require advanced engineering and often years of heavy-duty construction to build. They can also serve as miles-long passageways for everything from pedestrians and trains to the transportation of water for both consumption and hydroelectric power. Explore the interesting histories of the following six tunnels, which are some of the longest tunnels found in America. 

Snoqualmie Tunnel – Washington (2.25 Miles)

Entrance to the Snoqualmie Tunnel in Washington
Credit: davidrh/ Shutterstock 

Crossing under the majestic Cascade Range in Washington state, the Snoqualmie Tunnel is one of the world’s longest multi-purpose pedestrian and cycling tunnels. Part of the 250-mile-long Palous to Cascade State Park Trail, the tunnel was built between 1912 and 1914 as part of a railroad line from Seattle to Chicago. The last train passed through in 1980, after which the state acquired it and converted it into a recreational right-of-way.

Accessible from May through October, the tunnel offers adventurers the chance to embark on a mysterious hike or bike ride. Beside the flickering of visitors’ headlamps, the passageway sits in complete darkness. The temperature is notably cooler inside, and water often drips down through cracks in the roof. Other than a slight bend at the beginning, it’s a straight 2.25-mile journey toward the light at the end of the tunnel. Epic views of the Cascades and Granite Mountain await after reaching the western entrance. 

Anton Anderson Memorial Tunnel – Alaska (2.5 Miles)

Entrance to Anton Anderson Memorial Tunnel in Alaska
Credit: H. Mark Weidman Photography/ Alamy Stock Photo

Connecting the Alaska towns of Whittler and Bear Valley, the 2.5-mile-long Anton Anderson Memorial Tunnel (also known as the Whittler Tunnel) is the longest combined railroad and highway tunnel in North America. Inaugurated in 2000, the tunnel started out as a railroad tunnel in the 1940s, constructed to create a safe transportation route between Prince William Sound and the Turnagain Arm waterways. This once-dangerous route had previously been used by Chugach Eskimos and prospectors heading toward the Cook Inlet and Kenai Peninsula. 

Thanks to a unique design and intelligent engineering, the tunnel allows for a single lane of car traffic to drive directly over a train track. To do so, the original surface was dug up and replaced with 1,800 concrete panels. The structure can withstand temperatures as low as -40 degrees Fahrenheit, 150 mph winds, and avalanches of up to 1,000 pounds per square foot. It also features jet turbine ventilation, which cleans the air inside the tunnel between trips. Additionally, a system of traffic signals, gates, cameras, and a radar ensures that the tunnel is clear before traffic direction is changed. 

Cascade Tunnel – Washington (7.8 Miles)

Entrance to Cascade Tunnel in Washington covered with foliage
Credit: Wendy White/ Alamy Stock Photo

When train travel began in the Cascade Mountains, the Great Northern Railway needed a solution for transporting goods and passengers over Stevens Pass, located at an elevation of 4,016 feet in the Cascade Mountains. The first option was to lay switchbacks on either side of the pass. However, this proved too time-consuming, so a 2.6-mile-long concrete-lined tunnel was constructed between 1897 and 1900. The original Cascade Tunnel featured seven snow sheds, which helped to divert avalanches over the tracks.  

Increased maintenance costs, pollution issues, and a constant threat of avalanches and landslides convinced railroad officials to bore a new Cascade Tunnel. Opened in 1929 after eight years of construction, it remains the longest railroad tunnel in North America. The tunnel sits about two miles lower than its predecessor and connects the mountain towns of Scenic and Berne. During one stage of the construction, an estimated 1,750 workers were employed to ply away at the granite, toiling around the clock in three shifts, seven days a week, for 35 months. In 1956, a powerful ventilation system was installed to allow diesel-electric locomotives to pass through. 

Sudbury Aqueduct – Massachusetts (16 Miles)

View of Sudbury Aqueduct crossing river in Massachusetts
Credit: John Gaffen/ Alamy Stock Photo

The Sudbury Aqueduct runs from Farm Pond in Farmington, a western suburb of Boston, to the Chestnut Hill Reservoir. Constructed between 1875 and 1878, it features a horseshoe-shaped brick lining that measures 7.6 feet high and 8.5 feet in diameter. For an entire century, this 16-mile-long tunnel carried water from the Sudbury River to the city of Boston and its surrounding areas. While in operation, it was capable of transporting some 90 gallons of water per day. Taken out of service in 1978, the aqueduct is now part of the Massachusetts Water Resources Authority back-up supply. 

In 1990, it was added to the National Register of Historic Places, and much of the aqueduct’s route is now open as a public recreation space. It’s possible to see a number of landmarks that were once vital to the aqueduct’s operation, including Echo Bridge in Newton and the Waban Arches in Wellesley. These are accessible via a network of trails, complete with interpretive signs and wayfinding markers. 

Quabbin Aqueduct – Massachusetts (24.5 Miles)

Rowboats on rocky beach on the Quabbin Reservoir in Massachusetts
Credit: Gail Maloney/ Alamy Stock Photo

As one of the longest tunnels in the United States, the Quabbin Aqueduct plays a vital role in providing clean water to the Boston metropolitan area. It carries water for 25 miles between the Quabbin Reservoir and Wachusett Reservoir. Set about 800 feet underground, this rock-lined tunnel with reinforced concrete features 13 shafts, five of which are accompanied by attractive neoclassical-style headhouses. The section that stretches from the town of Ware to the Wachusett Reservoir was the first pressurized aqueduct constructed in the metropolitan Boston water supply system. 

Another notable feature of the tunnel is its ability to transport water in both eastward and westward directions, all by gravity. Water from the Ware River fills the Quabbin Reservoir via a shaft, where it remains for a period of five years. During this time, it enters into a natural purification process. The water is then sent on a westward journey via another shaft towards the Wachusett Reservoir. As well as contributing to the livelihood of the eastern Massachusetts population, the reservoirs at each end of the aqueduct are popular recreational sites. Both present opportunities for fishing, walking, and winter snowshoeing. 

Delaware Aqueduct – New York (85 Miles)

View of the Delaware Aqueduct crossing river in New York
Credit: Randy Duchaine/ Alamy Stock Photo

Constructed between 1939 and 1944, the Delaware Aqueduct is the longest tunnel of any kind in the United States, as well as the world’s longest water supply tunnel. This circular, concrete-lined tunnel, which ranges from 13.5 to 19.5 feet in diameter, consists of three individual pressure tunnels, each driven through bedrock at subterranean depths of between 300 and 1,550 feet. The entire tunnel takes water on an 85-mile journey from a collection of reservoirs on the west bank of the Hudson River to Hillview Reservoir in Yonkers, just north of New York City. 

On any given day, the Delaware Aqueduct carries up to 600 billion gallons of water to New York City. This amounts to almost half of the city’s daily supply. Situated in the foothills of the Catskill Mountains, the reservoirs that feed the aqueduct are open for recreational activities. Non-motorized boating is permitted on the Cannonsville, Neversink, and Pepacton reservoirs, while Neversink is known for its seasonal fishing. These tranquil bodies of water are important habitats for bald eagles, as they have some of the highest nesting densities found throughout New York state. 

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