If New York City and the surrounding areas want to continue growing, government must determine how to build a new subway extension in less than 100 years, according to the Regional Plan Association’s Fourth Regional Plan.

About a year after the opening of the Second Avenue subway’s first phase, the nonprofit association is publishing its plan Thursday, which includes proposals for eight new or extended subway lines to be built in the coming decades, as well as a unification of the region’s commuter rails — New Jersey Transit, Long Island Rail Road and Metro-North — under one system, called the Trans-Regional Express, or T-REX for short.

The 95-year-old association publishes such a plan every several decades to set the tone for planning discussions over the future of the tristate area. Its new report, a massive 351-page document, spans all sorts of issues pressing the region.

The MTA runs one of the largest subway systems in the world, yet more than a third of all New Yorkers don’t live within walking distance of a subway or train station.

The plan association’s subway expansions — some new ideas, others old — focus on connecting several key transit deserts, specifically neighborhoods considered low-income but with high enough housing density to support the trains, including southeastern sections of the Bronx and Brooklyn as well as areas of central and northeast Queens.

These new subways would intend to cut down some of the longest commutes in the city and reach what are now more car-dependent areas of the outer boroughs.

“The thinking that if you stop the development from occurring you will stop the rents from increasing is a false argument,” said Tom Wright, association president, during a briefing with reporters earlier this week. “The point is you have to put protections in place for those people. On a regional basis, you have to increase supply.

“This is going to be one of the political challenges over the next five, ten years I think,” he continued. “It’s figuring out how to put protections in place so those communities feel like they can accept growth without being pushed out, and figure out how to make that growth happen in a balanced way.”

While the Second Avenue extension cost $4.5 billion, the plan association also recommends overhauling the construction process at nearly every level — from environmental review, to procurements to labor regulations — to save costs and make these projects more realistic.

The MTA declined to comment on the subject of subway extensions or new lines before the publication of the plan Thursday morning.

Here’s a breakdown of new service proposals by borough:

Manhattan

Second Avenue subway: Extend the Second Avenue line from 96th Street past its next planned terminus of 125th Street and Second Avenue, to Park Avenue and then westward along 125th Street to Broadway. The idea is that in the three miles of expansion, the subway would hit underserved sections of Harlem while connecting to seven subway lines at four stations.

7 Line extension: Extend the 7 train from its current terminus at 34th Street down to 14th Street and Eighth Avenue, where it would connect to the L, A, C and E lines.

Brooklyn

Utica Avenue extension: Build a new subway under Utica Avenue, from Eastern Parkway to Flatbush Avenue, extending 4 train service by four miles.

Nostrand Avenue line extension: Build out the Nostrand Avenue line 2.7 miles south to Avenue Z, connecting 2 and 5 trains farther into Flatlands, Midwood, Marine Park and Sheepshead Bay.

Queens

Northern Boulevard line: Create a new 3.7-mile subway line running from 36th Street and Northern Boulevard to Willets Point, where it could either continue east to serve north Flushing and Mitchell-Linden or turn north to pass under Flushing Bay to College Point.

Jewel Avenue line: Build a 5.7-mile Jewel Avenue line that would branch off the Queens Boulevard line to the transit deserts of Pomonok and Fresh Meadows in central Queens.

Astoria line extension: Add a 0.8-mile extension to hook service closer to the East River at 21st Street and 20th Avenue. A new yard would be constructed on the northern side of Ditmars Boulevard along 20th Street.

The Bronx

Second Avenue extension: In addition to an expansion out west, the plan association calls for a northern expansion to the Grand Concourse at 149th Street to connect to the 2, 4 and 5 trains.