A easily-infuriated Muni rider was arrested Wednesday, after punching a fellow passenger in the head when her child bumped into him.

It all went down at 8:20 a.m., near 15th and Mission Streets. According to the San Francisco Police Department, a 34-year-old woman was riding the bus with her child in tow. Her kid, whose gender and age was not immediately known by an SFPD spokesperson, "accidentally bumped into" a fellow passenger.

That rider, a 34-year-old man, "became enraged," SFPD says, and punched the kid's mom "in the back of the head."

The blow caused "her to hit the front of her head on a metal railing," police say.

The rampaging man then fled the bus and ran south down Mission Street...and into the arms of police officers who'd been summoned to the scene.

The mad Muni rider was booked into San Francisco County Jail on aggravated assault charges. The injured woman was transported to San Francisco General Hospital for treatment of what SFPD says is non-life-threatening head trauma.