Upon entering the West 86th Street-Central Park West station on Saturday, I saw the dreaded yellow tape blocking the use of downtown trains. Isn’t the weekend subway schedule fun?

As I realized I’d be late for my niece’s birthday party in Brooklyn, an uptown train barreled into the station. A wiry guy with a scowl jumped the turnstile. In the train, he started baiting a chubby guy next to him.

“What the [expletive] are you looking at?”

Chubby guy didn’t respond, which emboldened the subway bully.

“You fat slob — let’s get off at the next station and I’ll kick your [expletive]!”

A mom sheltered her daughter as the nasty guy bellowed profanities.

Meanwhile, a well-dressed young man who looked like an actor sat quietly on the other side of nasty guy.

Suddenly, nasty guy rose and stormed up and down the car, slammed chubby guy’s shoulder, then leaned against the door. When the train pulled into the 116th Street station, actor-looking-guy rose to leave.

“You’re in my way,” he evenly told nasty guy, who spun around.

“Oh, you want some, too?” nasty guy said. “Get off and I’ll whip you!”

On the platform, actor-looking-guy whirled, put up his fists, and said, “Go ahead, big mouth!”

Through the window, I could see actor-looking-guy knew what he was doing. Was he a cop? A righteous avenger sent by God to right all wrongs?

I couldn’t wait to see subway avenger teach nasty guy a lesson, but as they circled each other, the train left the station. Aagh!

At 125th Street, I switched to the downtown D train.

A few days before, I’d read a report from the city Department of Investigation stating that “broken windows” policing, based on the idea that stopping smaller crimes prevents bigger ones, doesn’t work.

Police Commissioner Bill Bratton, who saw crime fall after instituting a crackdown on quality-of-life offenses in the ’90s, called the report “deeply flawed.”

Indeed, Saturday’s subway incident seemed a perfect example of Bratton’s concerns. If nasty guy had been apprehended when he jumped the turnstile, couldn’t the larger offense of terrorizing a subway car have been prevented?

Playwright Mike Vogel blogs at newyorkgritty.net.