Neat ideas are easy. Making them happen is hard. Like the 2nd Avenue Subway, 30 years behind schedule and finally — perhaps the more accurate word is “supposedly” — on track for completion.

This scenario might sound familiar:

New York lawmakers authorize a rail tunnel between Brooklyn and Staten Island. Construction begins but unexpected costs drive up the price and the money runs out. The project is scrapped fairly quickly, leaving big holes in the ground near what would have been the entrance to the tunnel. That was 1919. The holes were nicknamed “Hylan’s Holes” after Mayor John F. Hylan, who championed the project.

Nearly a century later, the prospects for big public works projects are so dire that even putting those holes in the ground sounds like an accomplishment. There was a time when politicians used big speeches to announce big ideas. But we’re at the point where ideas are so unlikely to become reality that rather than announcing new bridges, for example, Governor Spitzer used his grand State of the State address last week to announce he’s renaming a 72-year-old bridge after Robert F. Kennedy.

Big projects present something of a political dilemma for those in a position to make them happen. The lengthy development, approval, and construction process means that a politician who comes up with a brilliant idea will likely be out of office by the time the ribbon-cutting comes around. Tax cuts and school spending are immediate – and don’t require environmental impact statements and related obstacles.

Governor Pataki started the ball rolling on a couple of really good ideas. Now that he’s out of office, he’ll never receive any real credit. This presents an opportunity for Mr. Spitzer to make Mr. Pataki’s groundwork his accomplishments.

There are two projects in particular that stand out and that hold the promise of creating jobs, attracting tourists, and improving the lives of those of who live here and sometimes want to leave for a little while.

Plans for expanding the convention center named for Senator Javits and creating a grand train station named for Senator Moynihan represent opportunities for Mr. Spitzer to do good and thus look good during a time the word good doesn’t often appear near his name.

Just four years ago, plans for Javits seemed certain. City officials and lawmakers, state officials and lawmakers, the tourism industry, the hotel industry and construction industry agreed on a $2 billion plan for expanding Javits two blocks north to 40th Street. Then Mr. Spitzer entered office a year ago with plans for an even more robust expansion, nearly doubling the new square footage to 550,000 from 300,000. But last month the whole expansion plan collapsed, with the Spitzer administration citing the growth of the project’s cost to $5 billion – with nearly $1 billion in unanticipated repairs to the existing structure.

So now there’s silly talk of just building a whole new convention center from scratch, out in Queens. This would set in motion a whole new round of proposals, planning, and approvals (oh, and lawsuits and delays). There’s also no indication convention-goers and convention-throwers want to spend their time 12 miles away from the region’s core.

A good example of just how long delays can fester sits just a few blocks east of Javits, where the Farley Post Office was supposed to become Moynihan Station years ago. Amtrak, Long Island Rail Road, and New Jersey Transit riders have suffered through Penn Station’s dreary and decrepit underbelly since the original facility was torn down in 1963 to make room for Madison Square Garden. Senator Moynihan’s terrific idea was to convert the city’s main post office into a train station.

The New York Times first reported on delays for revamping the nation’s busiest transit hub in this dispatch: “After a delay of a year and a half caused by conflicts over personnel and rising budget estimates, city, state and Federal officials agreed yesterday to move forward with a $315 million project to build a new Amtrak railroad station in the General Post Office at 33d Street and Eighth Avenue in Manhattan.”

That was July 1997. The cost was $455 million. Opening day was to be in 2002.

More than a decade later, there are no shovels in the ground but plenty of ideas for expanding the project. Amtrak wants nothing to do with the new train station in the post office, meaning new renovations for Penn Station. Madison Square Garden wants to move across the street into a portion of the post office, leaving room for new skyscrapers. There’s talk of a huge retail mall, even a new Macy’s. The latest projected cost is $14 billion.

New York City needs a larger convention center and better train terminal in Midtown. These projects can be smaller than the biggest ideas but they need to happen.

Mr. Spitzer created plenty of problems for himself in his first year in office. If he can turn these much-talked-about projects into reality he will deserve credit for real accomplishments his predecessor failed to achieve.

Those revered ribbon-cuttings are just one election away. If Mr. Spitzer makes them happen he’ll deserve to win that election.

Goldincolumn@gmail.com