It always seems to start the same way.

I’ll beat you up!

A brush-fire confrontation between strangers in a subway loudly skyrockets toward hostility before anyone else is even sure who is yelling at whom, or why.

C’mon man! Do something about it!

A moment so familiar that other passengers hardly bother to look up from their phones or pause their conversations.

I’m gonna beat you up — only you. Just you.

But a fight on Thursday on a speeding A train in Brooklyn that started with those taunts did not end there. It continued to escalate before a rush-hour crowd, from words to fists to a blade to, finally, a gun.

The encounter came just over a week after New York’s governor took the extraordinary step of ordering the National Guard below ground to make the trains feel safer. The shooting undermined the city’s message that riding the subway is, statistically speaking, quite safe.

The episode fueled a sense of futility about a system that seemed to catch all the troubles from the city above — mental health crises, illegal guns — and squeezed them into crowded steel tubes.