New York has suffered 40 subway homicides since 2020, a five-fold increase compared to the post-millennial norm.

New York went through a similarly abrupt change in public safety underground before, in the mid-1960s — but took 25 years to fix it.

The fable of how New York achieved its miracle crime decline begins in 1990, with the stabbing death of 22-year-old Utah tourist Brian Watkins in a Midtown subway station, as he defended his parents from robbers.

Watkins’ killing shocked the city into cracking down on petty offenses — his killers had entered without paying their fares — before they escalated to violence.

But Watkins’ killing wasn’t shocking.

His was the 18th subway murder of 1990, and eight more people would be killed through that December.

For decades, New York had normalized subway violence, which had stolen the lives of students and grandmothers, executives and waiters.

Decades before Brian Watkins, as I write in my book, “Movement,” there was Andrew Mormile — whose murder was shocking.

From 1904, when the subway began running, until the mid-1960s, New Yorkers had ridden without fear.

In 1949, the subway carried nearly 2 billion riders with no homicides.

In the years before Mormile’s killing, though, crime was rising.

In 1964, riders and workers fell victim to 1,707 major subway crimes, nearly double the 923 in 1959.

Transit crime couldn’t be isolated from anxieties aboveground. Murders citywide had increased from 548 in 1963 to 636 in 1964.

In March 1965, on a Friday night, Mormile, a 17-year-old who loved animals, was dozing on a Brooklyn A train. Miscreants made their way through, shouting obscenities as they accosted passengers and groped girls. One assailant stabbed Mormile. The killer, Christopher Lynch, was also 17. 

Mayor Robert Wagner vowed to protect riders from “the mugger, the hoodlum and the young punk.” He ordered a patrolman on every train between 8:00 p.m. and 4:00 a.m. He more than doubled the transit police force.

By mid-1966, the Times reported that “murder, robbery, assault and other major crimes have been reduced nearly 60%.”

By the mid-1970s, the fiscal crisis forced the city to end full-coverage night patrols. Police gave out summonses for infractions like farebeating, but lawbreakers ignored them — police called them “disappearance tickets.”

The city lost control. In 1974, 7,626 riders fell victim to felonies. In 1978, the figure was 12,906. In 1972, six people were killed. In 1975, the number reached 17. Crime and disorder repelled passengers. In 1962, nearly 1.4 billion people rode the trains every year; by 1972, 1.1 billion did.

A new decade brought no relief. The 1980s began with 30 subway felonies in eight hours. Continued budget woes hindered Mayor Ed Koch’s crime-fighting efforts. New York had only 2,253 transit officers.

Transit crime was impervious to economic and population growth. During the 1980s, the population rose from 7.1 to 7.3 million. Yet an uptick in subway ridership didn’t reverse crime. In 1990, 18,324 New Yorkers became felony targets.

The resulting outrage and money created conditions for a new transit-police chief, Bill Bratton, to implement a “broken windows” strategy: deter small crimes, like turnstile-jumping, and prevent bigger ones. 

The quarter-million people entering the system daily without paying comprised 6% of riders. Most received a civil summons, as they had no arrest history.

These fare thieves were deterred from doing it again, though, under a new Bratton policy: hold them on a bus with criminals while police checked for warrants. Crackdowns on repeat offenders also ensued.

Broken-windows policing yielded results. By 1991, transit police — ranks boosted to 3,800 — made 17,492 farebeating arrests between January and November of 1991, compared to 8,679 in 1990. Serious crime fell 15%.

The trend held. Subway felonies were 70% lower than in 1990. That year, four people were killed on the subway and 6,218 fell victims to felonies — one-third of the 1990 total.

By 2017, felonies, at 2,474, were down to seven per day, from a high of 50. That year, no rider or worker lost his or her life to murder, even as transit-police ranks fell to 2,730 officers.

Safe subways were part of a safe city: In the late 2010s, New York’s murder level hovered around 300 a year, nearly 90% below the 1990 figure. Safe subways also underpinned a growing city, as ridership rivaled 1940s highs.

Now, we have lost much of this progress. It took nearly two decades for 40 people to lose their lives to homicide on the subway.

We’ve now compressed a generation’s worth of killing into four years — and with nine murders on the subway this year, this danger is not abating.

Assaults are 56% above 2019 levels. Fourteen percent of riders evade the fare, and a quarter of pre-2020 riders have deserted the system.

As in the 1960s, this abrupt shift is mirrored above-ground: City felonies are one-third higher than in 2019. Criminal justice “reforms” to avoid any penalty for lower-level crime make many summonses police hand out, and arrests they make, meaningless.

Will it take us 25 years to figure out the obvious this time?

Nicole Gelinas’ book on NYC transportation history, “Movement,” out Nov. 5, is available for pre-order.