Like so many busy New Yorkers in a hurry to get where they have to go, Sunando Sen peered out over the tracks on an elevated subway stop in Queens on Thursday evening, anxiously awaiting the next train.

What he did not see, the authorities said, was a woman approaching from behind who had been sitting on a bench and who had been heard mumbling to herself. Before Mr. Sen could react, the woman pushed him into the path of a No. 7 train roaring into the 40th Street-Lowery Street subway station in Sunnyside. Mr. Sen was crushed under the train.

As onlookers screamed, the woman fled the station down two flights of stairs. Her image was captured by a security camera as she ran down Queens Boulevard, casting a wary glance over her shoulder. She remained at large on Friday.

The seemingly unprovoked attack, the second time this month that a man was thrown to his death on the subway tracks, stirred some of the deepest fears of New Yorkers.