In just four and a half years, the proposed AirTrain to LaGuardia has grown from a $450 million idea of Gov. Andrew Cuomo’s to a $2.05 billion Port Authority turkey. Stop this train now!

Cuomo envisioned the new rail line, to be part of the $8 billion renovation of LaGuardia Airport, as bringing a 30-minutes-or-less ride to Midtown Manhattan. That fast trip, PA Executive Director Rick Cotton adds, would be “reliable and predictable.”

No way this plan does that: The 1.5-mile line would head away from Manhattan to connect to the No. 7 train and a LIRR branch at Willets Point. But it’s 20 subway stops to Midtown from there, and trains on the Port Washington branch don’t run often enough. The “fast trip” would take longer than your options now.

Not all the dough goes for the new line itself: The plan includes an LIRR station rebuild at $75 million, work at the No. 7 stop at $50 million and another $50 million to move the MTA’s Casey Stengel Bus Depot — plus construction of employee parking lots, a consolidated rental-car facility and a hotel.

Still, as the Manhattan Institute’s Nicole Gelinas notes, “You’re looking at over a billion [bucks] a mile” — a terrible standard for “large-scale infrastructure buildouts.”

Another bad sign: Cuomo announced recently that the MTA will be sharing the construction costs. Is his drive for total power over that agency really just a bid to loot it?

Tellingly, construction was originally supposed to start this year, but the Federal Aviation Administration just started its two-year environmental review.

That’s good, because the best time to put the kibosh on this mistake is before the first shovel hits the ground.