The Metropolitan Transportation Authority is studying a one-seat, 75-minute train ride to Stewart International Airport in Orange County, N.Y., from Pennsylvania Station that could help divert more passengers from the three overcrowded regional airports, stimulate the region’s economic growth, and increase real estate values upstate.

Transit officials say a three-mile spur off the Port Jervis line on Metro-North Railroad, which could cost more than $600 million to construct, could be an efficient way to attract passengers and airlines to the underutilized upstate airport, where the Port Authority of New York and New Jersey is expected to take control of operations in October.

The airport now is accessible only by country roads, and driving to the airport from Manhattan takes about an hour and a half without traffic. The former military base, located about 60 miles north of Manhattan, accommodated 300,000 passengers last year; transit officials estimate that its infrastructure could be expanded to process up to 10 million passengers annually.

With the city’s air routes as congested as its streets, business leaders say making Stewart International Airport accessible to New Yorkers is crucial if the city is to remain a financial and business capital. “Stewart Airport is the obvious solution, but only if it is serviced by fast rail into the city,” the executive director and CEO of the Partnership for New York City, Kathryn Wylde, said. The rail link “will pay for itself, in terms of the economic growth it will enable in the region.”

The MTA in July began searching for a consultant to conduct a $4 million, year-long feasibility study for the rail link, and it is eager to get the plan under way, a Metro-North spokeswoman said.

While the MTA moves ahead with the rail link project, some urban planners said the project could be costly and ineffective. “It’s going to be a real loser from an operating cost point of view,” a senior transportation fellow at the Regional Plan Association, Jeffrey Zupan, said. “It will have to run long distances and relatively frequent service, or people aren’t going to use it.” He said the real benefit of Stewart International Airport would be if it relieved the city airports of upstate residents who would no longer have to commute into the city to fly.

The Port Authority earlier this year purchased Stewart International Airport’s 93-year lease for $78.5 million from the National Express Corporation, to help relieve air and runway congestion at Newark International, La Guardia, and John F. Kennedy International airports. At those airports, more than a quarter of all arrivals were delayed last year, largely because of overcrowded airspace and runways. The three airports together fielded about 104 million passengers in 2006, and traffic is expected to grow to 150 million passengers by 2025, Port officials said. The National Express Corporation reportedly purchased the airport lease for Stewart in 2000 for $35 million.

One outstanding question is how a rail link would be funded.

The MTA, which owns the rail tracks, is taking the lead on the project, but in the past has put airport rail links low on its priority list, giving precedence to expansions of the system within the city, such as the Second Avenue subway and the East Side Access project.

Senator Schumer has been outspoken about raising federal funds for a rail link to Stewart International Airport, and the Port Authority, the Federal Transit Administration, the State Department of Transportation, or New Jersey Transit, which would run the trains, might also pitch in, a spokeswoman said.

The AirTrain to John F. Kennedy International Airport was ultimately paid for largely by passengers through an extra charge levied on airline tickets.

“The price tag for this project, which is three miles of track and some upgrades, are small potatoes compared to the Second Avenue subway and East Side Access,” a spokeswoman for Metro-North Railroad, Marjorie Anders, said. “It would be a very big return on investment.”

Mass transit investments historically help to increase real estate values, and a one-seat ride to the Hudson Valley could boost second home values. Commuters could also be willing to endure a 75-minute ride to the heart of Midtown.

But some experts said that the rail link would likely produce more urban sprawl. “You’re going to get a lot more driving, energy inefficient behavior, and land consumption, and people of limited means are not going to be able to live there,” Mr. Zupan said. “Is it going to be more Kentucky Fried Chicken and Dunkin’ Donuts, or can it be something better?”

The one-seat ride to Stewart International Airport could be completed only in conjunction with a Trans-Hudson Express Tunnel, a railroad tunnel that would connect New York and New Jersey and is slated for completion in 2016. Otherwise, passengers would have to transfer at Secaucus to catch the train. Another option would be to run the trains out of Grand Central Terminal.

The 11-mile AirTrain to John F. Kennedy International Airport cost about $173 million a mile, while a rail link to Stewart International Airport could cost more than $200 million a mile. The 1.9-mile AirTrain at Newark International Airport reportedly cost about $900 million to construct.

The MTA currently is working on five major construction projects competing for limited funding, in addition to a $6 billion rail link to John F. Kennedy Airport that is in the planning phases.