The Metropolitan Transportation Authority is considering a plan to invest almost half a billion dollars from an unforeseen surplus toward the construction of a platform over part of its rail yards on the far West Side of Manhattan, spurring development on the 13-acre site, including new MTA headquarters, officials announced yesterday.

The plan, unveiled in a memorandum released yesterday by the MTA’s executive director, Katherine Lapp, calls for using $481 million of a surprising $833 million surplus to build a platform that would encourage bids for commercial and residential development over the yards.

The announcement acknowledged a sharp turnaround from the $76 million the authority had anticipated, in a February budget report, that it would end the year with. Of the $833 million surplus, $365 million results from the authority’s share of tax revenues from a booming real estate market. An additional savings of $128 million on the authority’s debt payments is a result of lower interest rates.

Ms. Lapp said the transportation authority should use $12 million for security and service improvements, including $10 million toward overtime costs related to increased policing of the subways and $2 million for intensive track cleaning and station improvements.

The surplus led the authority to reassess its 2007 budget forecast from a $737 million deficit to a $461 million surplus, but deficits are still predicted in following years, and Ms. Lapp said the authority plans to pass incremental fare hikes on to passengers in 2007 and 2009, and every second year after that.

The recently conceived plan could help plug a gap left by the Jets when plans for a $1.6 billion, 75,000-seat stadium between West 30th and 33rd streets were quashed in June by a state board that declined to vote for a $300 million state subsidy for the development.

The MTA had included $1 billion in its 2005-09 capital plan from the sale of real estate assets, including the rail yards. With the Jets deal all but dead, that revenue stream became uncertain.

Ms. Lapp said the plan to rebid development rights for the yards should result in an offer close to that value, which the MTA had originally assessed the rights to be worth.

The president of the Jets, Jay Cross, contacted the MTA on Monday to ask for an extension until August 31 to pay a down payment of $50 million for the development rights. Mr. Kalikow said the authority granted the extension “with the understanding that the MTA would be free to pursue other options not involving the Jets.”

Ms. Lapp said the plan to build a platform was both “creative” and “bold” and was made possible by the “confluence of circumstances that occur once in a lifetime.”

She was referring both to the unexpected revenue and plans already set in motion, including a commitment reiterated by Mr. Bloomberg yesterday to extend the no. 7 line to the West Side; the development of the James A. Farley post office into a new railroad station to be named Moynihan Station, and the expansion of the Jacob K. Javits Convention Center. Much of the zoning for the development is also in place, a vestige of the bidding war between the owners of the Jets and Madison Square Garden over the right to develop the site.

“Like Rockefeller Center in the 1930s and Lincoln Center in the 1950s, now is an ideal time for a catalytic change in this neighborhood,” Ms. Lapp said.

Some of those who had opposed the Jets’ development lauded the MTA’s plan, which would allow the authority to move its headquarters to the new site and offload its current assets at 341, 345, and 347 Madison Ave. for millions more in cash.

“The big picture is that the MTA is clearly serious about maximizing the value of that site,” the president of the Regional Plan Association, Robert Yaro, told The New York Sun.

He said the “mixed-use” platform over the operating rail yards would cost about $388 million, allowing several hundred million dollars in revenue if the MTA sells it at its current asking price of around $1 billion.

One of the Democratic candidates for mayor, Gifford Miller, the City Council speaker, lambasted the MTA for not spending the unexpected windfall on subway infrastructure.

The district manager of Community Board 4, Anthony Borelli, said he does not believe the neighborhood of Hell’s Kitchen would fight the plan, as it had the Jets stadium. He also said he expects the MTA to include the community in the planning process.

An alternative and decidedly more conservative plan presented by Ms. Lapp yesterday called for the MTA to put the $481 million toward pension liabilities, which would translate into savings of $38 million annually.