We interrupt your celebration of the Second Avenue subway opening for an important announcement:

The “sick passenger” who caused trains to back up for miles on Dec. 16 wasn’t sick as you and I understand the word. To cite “The Godfather Part II” hit man Willie Cicci, that was no heart attack.

Nor was it a case of too much holiday eggnog. Although neither the MTA nor the NYPD will share data, the truth was obvious to this subway rider who’s used the system almost daily since 1972:

Many “sick passengers” who bring the system to a halt are homeless and/or psychotic people who do unpleasant things such as collapsing in their own excrement — and then require hour-plus efforts by cops and EMS workers to remove them from trains.

The MTA does a heroic job keeping the system efficient and safe as ridership continues to soar. But the 662.75 “mainline” track miles and 472 stations are increasingly full of disruptive and dangerous head cases.

You don’t like “head cases?” All right, then: insane people. Not only do some loonies knife and push normal riders onto tracks, they bring crowded trains to a halt just for the heck of it.

On Dec. 16, shortly before 11 a.m., someone had the fun idea to repeatedly pull the emergency-brake cord on a southbound F train stopped in the Rockefeller Center/47th-49th Street Station.

I was among hundreds of passengers on an M train immediately behind the F. The M remained stopped, just short of the station, for 70 long minutes — I clocked ’em, 70 — before the F was able to leave.

Chaos naturally ensued. On one of the holiday season’s busiest shopping days, the MTA rerouted all downtown-bound F and M trains from Sixth Avenue to the Eighth Avenue line.

An MTA employee on the Rock Center platform described the brake-activator to me as a “nut job.” There’s no way to know for sure. The MTA wouldn’t specify “symptoms” — consistent with a pattern of obfuscation regarding underground madness.

The MTA and City Hall would rather we didn’t know how many serious delays are caused not by signal and switch problems but by the lawless antics of the homeless, addicted and/or schizophrenic.

Not only do some loonies knife and push normal riders onto tracks, they bring crowded trains to a halt just for the heck of it.

In November 2015, The New York Times reported that sick passengers caused 3,000 train delays each month of that year, up from 1,800 a month in 2012. No doubt some of them suffered from normal physical ailments. The MTA declined to give us a figure for this year. The NYPD wouldn’t say how many of roughly 1,700 “aided cases” (medical emergencies to which cops responded) it handled citywide in 2016 were in the subway.

At least the F-train cord-yanker apparently wasn’t violent — unlike the schizoid woman who pushed another woman to her death on the Times Square tracks on Nov. 8, or the homeless guy who pushed a woman down the stairs at Forest Hills on Dec. 24 or the “disturbed” homeless woman who slashed a fellow rider on a No. 4 train in East Harlem on Christmas.

Although my M-train incident wasn’t half as scary as the evacuation due to fire of 1,000 riders from D, E and F trains on Christmas night, it would have been a nightmare had the mercifully calm and orderly front car in which I rode held panhandling acrobats, a foul-smelling, near-dead person, or a raving maniac.

The conductor did his level best to tell us what was going on. But he was at the mercy of shape-shifting misinformation that reflected institutional unwillingness to say, simply — “Sorry folks, there’s a crazy person on the train in front of us.”

The conductor first told us that the F train’s emergency brake had been pulled, and the F would move once EMS, which was on the scene, removed the “immobile” passenger.

Fifteen minutes later came word that EMS in fact had yet to show up. Then, the M would “proceed with only the first car into the station,” where everyone could exit through the first car. That was soon countermanded by orders to reverse direction and “return to Fifth Avenue” — which didn’t happen either.

Who to blame? Not so much the MTA, which takes safety very seriously. The fault lies with City Hall, which rolled out the red carpet to the “homeless” from everywhere and gave them free rein on the street — and beneath it, where the lunatics now rule the rails.