The Gateway trans-Hudson rail tunnel plan is poised for another giant step this week with an expected agreement for an interim body to kick-start the $20 billion job — in advance of a full-fledged development corporation to be established later this year.

There’s still a long way to go, but Sen. Chuck Schumer (D-NY) enthusiastically cited the “memorandum of understanding,” or MOU, as part of “major progress in the past few months” on the critically needed Amtrak tunnel.

The MOU will bring together the states of New York and New Jersey, Amtrak, the US Transportation Department and New Jersey Transit on an “interim governance” panel until Gateway Development Corp. is launched.

Gateway will create a new, two-track rail tunnel beneath the Hudson River between New York and New Jersey for Amtrak and NJ Transit.

Although it sounds less sexy than other major proposed projects, such as a modernized LaGuardia Airport, “it is the most important infrastructure project in the metropolitan area,” Schumer told us again this week — echoing his earlier warnings that, “Without it, our entire area comes to a grinding halt and we plunge into recession.”

The crumbling and overworked two-track tunnel now in service is more than 100 years old. It was severely damaged by Superstorm Sandy and might be useless in as little as seven years.

Govs. Andrew Cuomo and Chris Christie previously, and quietly, backed off earlier statements that Gateway should be managed entirely by the Port Authority — the scandal-ridden and internally divided bureaucracy where Executive Director Pat Foye and Chairman John Degnan exchanged insults at a public board meeting last week.

The GDC instead will have four members — one from each state (likely drawn from the PA), the federal DOT and Amtrak — who together will choose an executive director to manage the massive project and report to the board.

The GDC can tap funds from various government sources more swiftly and efficiently than the states or agencies could do individually. Schumer said the GDC “will be uniquely able to receive Amtrak, federal and state funds. We’re at a technical phase of dotting i’s and crossing t’s, and should be done in two months.”

The temporary body to be set up first will have “broader membership” than the GDC, with an executive committee including representatives from Schumer’s office and for Sen. Cory Booker (D-NJ).

They’ll get the ball rolling on environmental permits and preliminary engineering, construction and financing.

However, sources noted that establishing the full-fledged GDC requires “very complex” legal and financial language to make sure it can fulfill its mission. Part of that is to follow through on an agreement made in November that the total Gateway cost would be split 50-50 between Washington and the two states.

Sources said it’s crucial to set up the GDC and to begin applying for certain funding before a new administration takes over in Washington in January. “It’s a tight time frame,” one insider acknowledged.

In other Gateway news, Amtrak and the PA have agreed to split the $70 million cost of preliminary engineering, and US Transportation Secretary Anthony Foxx pledged to commit resources and accelerate environmental reviews.

Last winter, a federal transportation act championed by Schumer freed Amtrak to apply some of the $400 million in profit it makes from the Northeast Corridor — previously used to subsidize the system’s many unprofitable lines — to Northeast infrastructure capital projects.

Nothing is certain, but Schumer said, “I hope actual construction can start in the second year from now. That’s an optimistic but very possible scenario.”

Mysteries Solved Dept.:

What’s that ugly orange material in 432 Park Ave.’s open-air “windows”?

As long as it remains, the super-tall condo tower by CIM Group and Harry Macklowe has an unfinished look.

Turns out it’s temporary safety netting. Once it’s gone from the uninhabited floors — which are mostly hollow for wind to pass through as a stabilizing measure — large spherical metal drums inside the bays will be dramatically lit at night.

Is Whole Foods ever going to open its promised stores in Harlem and near Bryant Park?

You bet your organic banana.

It’s been so long since leases were announced for 100 W. 125th St. (in 2012) and at 1095 Sixth Ave. at 42nd Street (2013), some snarky brokers and readers have asked us if the deals fell through.

Actually, construction is ongoing at both locations. Whole Foods spokesman Michael Sinatra said both will open in late 2016 or early 2017, “likely within a few weeks of each other.”

Although there have been reports that the Midtown store might have a “different concept” and possibly a different name, Sinatra said both it and the Harlem edition will have “no difference in terms of name or branding — they are Whole Foods Market stores.”