The concept is hardly new; it has been debated for at least a century, with no discernible results.
A 1910 article in The New York Times, under the headline “Transit Outlook Bright in Brooklyn,” said “a strong movement” was afoot to construct it. Another effort was made in 1928. More recently, an attempt in the 1970s failed after a City Council member from Brooklyn complained that the area did not have a large enough population to support the line.
Nonetheless, the de Blasio administration placed the concept back in the spotlight, suggesting that the area was a prime spot for expansion because it is one of the most crowded parts of the city without direct access to the subway.
The mayor’s request for the Metropolitan Transportation Authority to examine bringing No. 3 and 4 train service down from the Eastern Parkway branch, through East Flatbush, came as a surprise to many.
“No one expected this,” said Mitchell Moss, the director of the Rudin Center for Transportation at New York University. “It’s refreshing to see a proposal to extend mass transit into areas of Brooklyn that are transit-deprived. It’s obviously an idea that will take more than a decade to be carried out, but you have to start with an idea.”
Transit advocates quickly pointed out that Mr. de Blasio had not offered a financing plan for an extension, and that the authority already had a $15 billion gap for its five-year capital plan for improvements to the aging system.
“The issue is: Where’s the check?” Gene Russianoff, a staff lawyer for the Straphangers Campaign, an advocacy group, said of the Utica Avenue idea. “It’s only credible if the city is willing to help finance it.”
A spokesman for the authority, Adam Lisberg, declined to comment specifically on the Utica Avenue study. Instead, he released a statement drawing attention to measures in the capital plan that would expand and improve service, including modernizing the subway signal system and proposals for improved bus service.
“The M.T.A. and the City of New York are aligned on the importance of mass transit to keep the city thriving,” Mr. Lisberg said.
The authority has several other major projects in the works: The first phase of the Second Avenue subway is scheduled to open at the end of next year, and the East Side Access project, which would bring the Long Island Rail Road to Grand Central Terminal, is supposed to be completed in 2022. And in January, Gov. Andrew M. Cuomo proposed an AirTrain to La Guardia Airport.
Jumaane D. Williams, a City Council member whose district includes Utica Avenue, said the area needed better transit options and the subway extension sounded good in theory. He also noted that the Second Avenue subway sounded good — in theory.
“I’d like to talk about it more when the Second Avenue train is completed,” he said with a tone of skepticism. “But I’m happy the mayor is talking about transportation issues in South Brooklyn, because too often it’s left out of the discussion.”
Mr. Russianoff agreed, saying that Mayor Michael R. Bloomberg had instead prioritized extending the No. 7 line to the Far West Side of Manhattan — a project that has been delayed but may finally open this summer. (Mr. Bloomberg directed the city to pay for the $2.4 billion project, the first subway extension paid for by the city in more than 60 years.)
“Utica Avenue is much more in de Blasio’s theme of providing service more equitably and keeping in mind the ordinary New Yorker,” Mr. Russianoff said.
The dream of a Utica Avenue line has had plenty of false starts.
Its plight was outlined in the 2013 book “The Routes Not Taken: A Trip Through New York City’s Unbuilt Subway System,” by Joseph B. Raskin, a former New York City Transit employee. Past proposals would have connected it to the No. 3 and 4 lines, the A and C lines, or even to Lower Manhattan. “It’s always been to me one of the lines that should have been built all along,” Mr. Raskin said. “The question was always what line is it going to plug into — and the cost.”