With uncertainty looming over who will pay for the extension of the no. 7 subway line to the far West Side if it exceeds its current cost estimate, alternative plans using existing transit infrastructure have come back into view.

One plan involves using existing Long Island Rail Road tracks from Pennsylvania Station to the West Side rail yards for a subway shuttle that would connect straphangers to the planned New York Sports & Convention Center. Proponents said it would provide a majority of subway riders with easy access to the far West Side for around $150 million, a fraction of the extension’s current price tag of $2.1 billion.

Another idea is to utilize an unused, existing track below Eighth Avenue and then connect west from Penn Station.

Yet another involves using existing Amtrak tracks that connect Tenth Avenue and 41st Street to 35th Street and Eleventh Avenue, two blocks north of the proposed Jets stadium.

Though all three ideas have already been rejected, transit experts continue to urge the Metropolitan Transportation Authority to think more broadly about how the no. 7 extension fits into the larger vision of increasing subway capacity and creating a regional transportation network.

The no. 7 extension has long been championed by Mayor Bloomberg and Deputy Mayor Daniel Doctoroff as a key element of their plan to revitalize the far West Side. In addition, if the city wins its bid to become host of the 2012 Olympics, the extension would provide direct access for athletes from the planned

Olympic Village beside the East River in Queens to the new stadium, which would be the main Olympic venue.

Compared with other expansion projects, the no. 7 extension bears the lowest return for its costly investment, according to the president of a transit think tank, the Institute for Rational Urban Mobility. George Haikalis, who proposed the idea of a shuttle using existing Long Island Rail Road tracks at Penn Station, said his idea would serve all riders except those on the Lexington Avenue line, especially if an existing pedestrian passageway were opened to connect Herald Square to Penn Station.

“If you get 80% of the access with a small amount of investment, we should think seriously about that,” Mr. Haikalis said.

Critics of the no. 7 extension also note that, at least initially, it will add only one station to the line: at Eleventh Avenue and 33rd Street. Another station could eventually be built at Tenth Avenue and 41st Street.

Before endorsing the no. 7 extension as the best plan, the Department of City Planning and the MTA reviewed a proposal like Mr. Haikalis’s in November and rejected it, along with several other ideas, in their environmental impact study.

Last fall, planning officials said the shuttle idea meant reducing from 30 to 28 the number of tracks used by the LIRR, whose officials say it is already operating beyond capacity. Under federal regulations the shuttle can operate only six times an hour in each direction.

Mr. Haikalis said focusing on another expansion project known as East Side Access, a link between the LIRR and Grand Central Terminal, would alleviate overcrowding and help bring riders from the East Side, accomplishing two goals of the no. 7 extension.

Until the extension of the no. 7 line was cited Thursday by the board of the MTA, which voted unanimously to approve the Jets’ bid to develop their stadium above the western rail yards, it was regarded by many transit planners and elected officials as a lower priority than the construction of the Second Avenue Subway and East Side Access.

With the bid, though, came the promise by Mr. Bloomberg to contribute $2 billion for the extension, making it especially attractive to the MTA board. The cost of the extension has grown by 50% from the initial estimate, and transit experts say it is sure to rise further.

In the past, the MTA has board has said it would not finance the extension, though it has spent several million dollars on engineering to explore the feasibility of the idea.

The associate director for the government-appointed group that represents subway riders, New York City Transit Riders Council, William Henderson, said that among the reasons planners rejected the notion of extending the subway south under Eighth Avenue is that delicate construction would be necessary, including work below the construction site of the future New York Times building between 40th and 41st streets.

“The MTA favors tunnel-boring machines rather than using existing infrastructure,” Mr. Henderson said.

The president of the MTA’s Capital Construction department, Mysore Nagaranja, rejected that view, saying simply that alternatives were not pursued “because they were not good options.”

The major drawback to using existing Amtrak lines is that unless a tunnel is dug beneath the track, the trains would have to cross paths, presenting an almost impossible engineering challenge and potential dangers, one city planning official said.