While it is being floated as a replacement for a cancelled New Jersey Transit train tunnel into Penn Station, the Bloomberg administration's plan to extend the No. 7 subway line to New Jersey would benefit groups that didn't have as much to gain from the rail link.

New Jersey residents with jobs on the East Side of Manhattan could take the No. 7 train straight to Grand Central Station, slicing their commute times. Queens residents would have an easier trip to Newark-Liberty International Airport, while New Jerseyites could take the subway directly to buses that go to LaGuardia International Airport.

Then there are the sports fans. New Jersey-dwelling Mets fans would get a one-seat ride from Secaucus to Citi Field in Flushing. And Queens-dwelling Giants and Jets fans could hop a subway to Secaucus, then connect with a shuttle trains to the Meadowlands football stadium.

Of course, Mayor Michael Bloomberg's administration is only exploring the subway extension, the cost of which it pegs at $5.3 billion—about half the cost of New Jersey Transit's Access to the Region's Core tunnel.

New Jersey Gov. Chris Christie killed the tunnel late last month, saying his state couldn't bear the risk of potential cost overruns.


New Jersey Transit has greatly increased the number of trains into Penn Station in recent years, but the current tunnel can't handle much more. Building a new tunnel to a terminal adjacent to Penn Station meant essentially doubling down on the same strategy, just adding more trains.

Extending the No. 7 subway train is something very different. It would directly hook New Jersey into the huge New York subway system, bypassing Penn Station.

It could have a big effect on Manhattan. The subway extension would make it easier for the masses of northern New Jersey to get to the big parcels slated for development on Manhattan's far West Side.

It could also provide new life for development in areas like Queens's Long Island City neighborhood, which saw a building boom before the real estate bubble burst but has stagnated since the recession. It would suddenly be five subway stops away from New Jersey.

"If it's as inexpensive as the city believes it is, it's a phenomenal plan," said Steven Spinola, the president of the Real Estate Board of New York.

There's no money immediately available for the No. 7 proposal. Metropolitan Transportation Authority Chairman and Chief Executive Jay Walder said Wednesday that he's focused on making sure the megaprojects his agency is already working on—the extension of the No. 7 train to the Javits Center is among them—get finished. He said the MTA didn't get involved in the plan being circulated by the city until Tuesday afternoon, hours before it became public. The legalities of running a New York City subway across state lines are complex. And New Jersey officials haven't been involved in the planning.

"Obviously, anything we can do for another transit connection is exciting, but the devil's in the details," said James Simpson, the New Jersey transportation commissioner. "I think it's a grand idea, but it would be nice if somebody from New York called New Jersey and let us know, if people are serious about a project like this."

Despite all that, a subway link to New Jersey continues to be a dream of transit planners—and they insist that it's not an impossible one. Officials on both sides of the Hudson River have made clear that the need for improved mass transit between New York City and New Jersey remains, despite the end of the ARC tunnel project.

"This is not an unreasonable idea in the absence of ARC," said Jeff Zupan, a senior fellow at the Regional Plan Association. "It shouldn't be just dismissed as somebody came up with a harebrained idea."

Many of the northern New Jersey mayors gathered in Atlantic City for the annual New Jersey League of Municipalities convention were intrigued by the proposal. Access to jobs in Manhattan is a crucial issue for their constituents.

Adam Schneider is the mayor of Long Branch, on the Jersey Shore. His town is designing a ferry project for commuters to Manhattan, but said a subway connection for riders leaving the town on the New Jersey Transit line would be helpful.

"I saw it and I thought, what a good idea," he said. "The tunnel I had mixed feelings about. It's a lot of money, it didn't seem all that well planned and all that well designed, but you need public transportation that has been ignored for so long."

—Eliot Brown contributed to this article.

Write to Andrew Grossman at andrew.grossman@wsj.com