“What made this nation this nation? What made this state this state? It’s what we built. It’s what we created.”

Thus proclaimed Gov. Andrew Cuomo this weekend, after numerous visits to make sure New York City’s most elusive public construction project was finally, finally on the verge of becoming real: the Second Avenue subway.

After nearly 100 years, the day of reckoning has finally come. The MTA is racing against an end-of-year deadline to finish the first phase of the project, the first line extension in decades besides the new Hudson Yards 7 stop.

MTA chairman Thomas Prendergast said Monday that he is “cautiously optimistic” that the deadline will be met. That kind of will-they-won’t-they has brought Prendergast’s boss, Cuomo, down from Albany, for multiple site visits where, as he said on The Cats Roundtable with John Catsimatidis, he was making his presence felt by the contractors. He was throwing his weight around. Put another way: There better not be any screwups under his watch because a new age of public construction had dawned in New York State.

Over the past year and a half, Cuomo said he had “taken a personal hands-on approach, and that’s the only way you get things built.” Without making any unbreakable promises, he allowed that if he had to “bet . . . and it was even money, I would bet that we make it.”

So Cuomo has now directed all eyes to the east side of Manhattan, to see whether the first trains will run on fresh tracks by New Year’s Day. But whether they do, what happens next?

The once and future subway line

First proposed during the Roaring 20s, the Second Avenue line was meant to be a muscular addition to the system’s capacity, bustling with passenger activity and energy from Hanover Square to East Harlem or beyond.

That was the plan. Then the 20th century happened.

Plans were shelved during the Great Depression and World War II. Federal funds were allocated in the 1970s, but the city’s fiscal crisis put the project on hold again.

After nearly 100 years of stops and starts and delays, we’re left not with a bold new line running the length of the world’s busiest island, but a $4.5 billion extension of the Q line three stops to 96th.

That is what all the hullaballoo is about, the peerless victory that a determined Cuomo and others are edging toward declaring.

To be fair, this is certainly a monumental achievement in MTA-land. Life will be more convenient for the 200,000 daily riders the line is likely to serve, and the 4-5-6 line — currently the city’s busiest — will finally have some breathing room.

But there will be plenty of work left to do.

What’s next?

Earlier this year, the MTA dedicated funds for the second phase of the line up to 125th. Officials and residents will have to continue to be as diligent to see that the next 30 blocks don’t take a century — and that the extension south is finished sometime before global warming eats away at Manhattan’s tip.

There are other megaprojects for NYC to turn its attention to as well — starting with the $10 billion East Side Access project which includes nearly two miles of new tunneling to bring the LIRR to Grand Central Station, reshaping travel to and from LI.

As NYC’s population expands and the subways continue to be the place of first and last resort for city travel, the MTA also will need to devote resources to everyday commutes.

Statistics relased at Monday’s MTA board meeting show many lines are racked with delays; car-related delay incidents have become more common over the past year. MTA spokesman Kevin Ortiz says one reason is the aging train fleet, with some cars from the so-called “legacy fleet” dating to the 1960s. Some new cars are on order.

Ortiz points to train failure numbers from the ’80s, when failures were nearly twice as likely following a period of disinvestment in the system.

That tradition of disinvestment can’t be allowed to continue, as we head into another period of growth for the subway system.

Moving the goalposts won’t provide any actual relief for commuters who breath down each others’ necks every morning and night. Cuomo and the MTA will need to be vigilant about projects large and small, beyond Second Avenue.