A Metropolitan Transit Agency worker was punched in the face after stepping in to break up a subway argument Thursday — just steps from where a sword-wielding wacko murdered an innocent man the week before, cops said.

MTA track worker Michael Grosso, 59, was on duty a little after midnight, working on the southbound A/C line in the Jay Street station, when he spotted a man arguing with a passenger on the platform and decided to step in, police sources told The Post.

However, Grosso quickly became the next target. 3

The suspect — who police described as a black male in his 40s dressed in all black — turned on Grosso, balling up his fist and punching Grosso in the left side of the face, police said.

The suspect then bolted topside escaping into the night.

Despite the shocking att

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