The woman appeared to be mumbling to herself and pacing, witnesses told police, before she approached the man from behind on the platform of an elevated station in the borough of Queens and shoved him onto the subway track Thursday evening.
She ran from the station to the street in a scene caught on video footage that the police released on Friday as they searched for the unidentified woman, who, they said, was last seen wearing a ski jacket along with red and gray Nikes.
The death in Queens was the second time this month that a New York subway rider was killed after being pushed onto the tracks - a chilling notion for anyone who uses the city's transit system - and came just ahead of the New Year's holiday in a city choked with visitors.
"To say that it's only two in, you know, a long period of time doesn't help either person," New York Mayor Michael Bloomberg said on Friday on WOR Radio 710 AM's "The John Gambling Show."
Police said it remains unclear whether the woman being sought knew the victim, who they initially had trouble identifying because his body had been so badly mangled by the train. He has yet to be named.
In New York's subway stations, some took added precautions Friday, standing a few extra feet away from the platform edge. Commuter Chloe Morris, traveling from New Jersey, said she prefers to sit on a bench rather than stand on the platform while waiting for a train.
"I don't come close to the edge until a train comes," Morris said as she waited in the Times Square station. "There's too many crazy people in the world."
On December 3, Ki-Suck Han was killed after being shoved onto subway tracks in Manhattan as a train entered the 49th Street station near Times Square. A suspect, Naeem Davis, has been charged with second-degree murder.
Including Thursday's incident, 139 people were struck by New York City subway trains so far in 2012, 54 of them fatally, a Metropolitan Transportation Authority spokesman said on Friday, adding that the tally was preliminary and subject to change.
Police Commissioner Ray Kelly indicated the department would not be adding more police to platforms anytime soon.
"We think we are properly deployed in the transit system," he said at a news conference on Friday.
One solution might be installing safety doors along subway platforms that block access to the tracks until a train arrives, something in use in several major cities around the world, said New Yorker Tom Walker as he waited for a subway.
New York's subway system, which is more than 100 years old and is one of the world's busiest, does not have barriers between the platforms and the tracks.
"It's an antiquated system. Of course people are going to fall in," Walker said.
(Additional reporting by Ian Simpson; Editing by Paul Thomasch and Leslie Adler)