A monorail line between Frederick and the Shady Grove Metro station in Montgomery County could be built for $4.4 billion and allow passengers to reliably make the 28-mile trip in 42 to 46 minutes, according to a study by the Maryland Department of Transportation.

While the agency found a monorail to be a “viable” public transportation option for the Interstate 270 corridor, it also cited a “concern” that it would draw passengers from other transit systems and barely reduce traffic on Interstate 270.

Montgomery and Frederick officials have long sought to reduce congestion in the I-270 corridor, where the highway, at least pre-pandemic, was jammed most mornings and evenings. The corridor is Montgomery’s hub for biotech companies, and the highway carries commuters between more affordable homes in farther-out suburbs and jobs in Montgomery, Washington and Northern Virginia.

Meanwhile, companies in the corridor say they need better transit options to attract workers, particularly millennials who want to avoid driving.

Of the 34,800 to 47,800 people expected to ride a monorail daily, based on its speed and frequency, nearly 10,000 would come from other transit systems, including MARC commuter rail and express commuter buses. A monorail would attract about 10,300 new transit trips daily, which would take that many vehicles off I-270 and other roads in the corridor — a less than 1 percent reduction in future traffic, the study found. The remaining riders would come from anticipated population and job growth.

“The [monorail] ridership has been determined to be a concern due to the decrease in ridership anticipated from the other transit services and the overall lack of reduction of single-occupancy vehicle trips from ­I-270,” the study said.

More development would have to be clustered around potential station sites to generate ridership, and motorists would need a “behavioral shift” to switch to mass transit, the study said.

Monorail advocates say they question some of the findings, particularly the assumptions underlying the ridership calculations. However, the overall conclusion that a monorail would be feasible, they say, shows it deserves serious consideration as a way to expand mass transit.

“They’ve confirmed that this is a viable transit mode,” said Montgomery County developer Robert O. Eisinger, the proposal’s primary champion, who has funded his own monorail consultant studies via his High Road Foundation. “This is not Disneyland. This is not Mickey Mouse. If nothing else, this says when you’re looking at traffic congestion solutions, this has to be considered.”

Along with some jokes, Eisinger’s idea has drawn attention at a time when some state and local officials say MDOT’s plan to relieve traffic congestion on I-270 and part of the Capital Beltway by adding high-occupancy toll (HOT) lanes gives short shrift to mass transit. MDOT rejected the idea of expanding the MARC commuter rail system as part of its “traffic relief plan” but has said buses using the congestion-free toll lanes would become more reliable and attractive.

MDOT also has said Transurban, the Australian toll road operator, and Macquarie, its partner investment bank — the companies it has selected to develop the first phase of its HOT lanes plan — have estimated they could commit at least $300 million over a 50-year partnership to mass transit systems in Montgomery.

Eisinger said he agreed with much of the state study, except for ridership forecasts and predictions of how much traffic a monorail would take off roads. He said ridership forecasts were “very, very conservative” because they were based on assuming a monorail would travel below 65 mph. The state report says the study assumed an average speed of 35 mph for more conservative ridership forecasts and to account for the fact that trains need to stop at stations and slow down on curves.

Eisinger questioned whether the Washington Council of Governments — which analyzed potential ridership in the study — used old forecasts for future development, rather than assuming that high-density development could be clustered around monorail stations. He also questioned why the study didn’t specifically examine how much traffic would be taken off I-270 rather than all roads in the area.

Drawing passengers from other transit systems, especially those that travel more slowly, would better serve riders and save state and local governments money if some redundant express bus routes could be eliminated, he said.

The state did the study at the direction of the Maryland Board of Public Works, which required it when the board approved the HOT lanes plan. The next step would be a more detailed environmental impact study, which would be required to receive federal construction funding.

MDOT spokeswoman Erin Henson said the agency will seek feedback on the feasibility study from Montgomery and Frederick counties and see whether either includes a monorail in its annual letter, expected this spring, outlining local transportation funding priorities.

The study found that an “aggressive” schedule would allow a monorail to be built in about five years. It would have six stations: the Frederick MARC station, Urbana, Clarksburg, Germantown, Metropolitan Grove and the Shady Grove Metro station in Rockville.

The biggest hurdle: No one has identified a way to pay for it, including its ongoing operating costs, considering transit lines typically require government subsidies. Eisinger said the line could be built and operated via a public-private partnership. His consultant’s analysis found that a monorail priced similarly to Metro would collect more than enough fare revenue to cover the projected annual operating and maintenance costs.

Eisinger said he believes property owners around station sites would donate land in exchange for being allowed to build at higher densities. He said his company, Rockville-based Promark Partners, owns property across the Washington region but not at any potential monorail station sites.

As part of the study, MDOT found 57 monorail systems worldwide, including eight in the United States.

Most U.S. monorails run in short loops around amusement parks or between airport terminals. The most well-known systems are privately funded lines linking Disney World resorts and Las Vegas casinos. Seattle operates a one-mile monorail built for the 1962 World’s Fair.

While a more detailed environmental impact analysis would be required, the study found that building monorail stations and maintenance facilities would affect up to 27 acres of private residential and commercial property. Most of a monorail could be built within the highway right of way.

The study noted that ridership forecasts also would be affected by how quickly, and how much, transit ridership rebounds after the pandemic.