It is a hidden reality of the New York City subway system, and perhaps mass transit systems everywhere since the first trolley car took to the tracks. It begins with a pinch or a shove, someone standing too close. But it can be much worse.

This week, as the Police Department announced the arrest of 13 men charged with groping and flashing women in the subways, women around the city nodded. Yes, they said, this had happened to them. Yesterday. Last month. Last fall. Twenty years ago.

"Every girl I know has at least one story," said Barbara Vencebi, 23, a studio photographer standing outside the No. 6 train station at 116th Street in East Harlem yesterday.

It is a crime abetted by the peculiar landscape of the underworld that is the subway system, by the anonymity of a crowded car where everybody is avoiding eye contact. And by the opportunity for a quick escape at the next stop, to disappear behind a pillar, into a tunnel, up an escalator.

An impromptu survey of riders during the morning rush yesterday found that, for many women who have experienced it, the worst part of the crime is the sense of helplessness. What is the right way to react to a humiliating, but not life-threatening, situation? Should you announce to an entire car of strangers that you have just been violated?

Most of the time, the women said, they seethe inwardly but say nothing.

"I looked back and I couldn't do anything because a lot of people were behind me," said Suany Baca, 32, a waitress who was going up the stairs at 86th Street in the No. 6 train station last November, when she was groped by a man who passed her going down.

"I pretended like it didn't happen," she said. "I don't know what they get out of it."

Those who single out women on the subways do not care about race, if yesterday's interviews were any indication — black, Asian, Hispanic and white women all had stories to tell. But they do seem to discriminate by age.

Most of the women who reported recent incidents were in their 20's and younger. But the experience, women said, is so universal, and so scarring, that they continue to feel paranoid and to put on their body armor — the big bag, the bad face — no matter how old they get.

Women know the drill. Just as some men reflexively check to see if they have their wallets on a crowded train, women check their bodies.

Pull in your backside and your front. Wedge a large bag for protection between yourself and the nearest anonymous male rider, who might, just might, be planning something. Put on your fiercest face, and brace yourself for contact that seems too deliberate to be accidental, too prolonged to be random.

And not just in New York. Mexico City and Tokyo have reacted to subway gropers by instituting all-female subway cars. But as one New York woman said yesterday, wouldn't that make a nice target?

The crackdown in New York followed a number of highly publicized cases in which women helped the police arrest flashers by snapping pictures of them with their cellphone cameras.

Some women said yesterday that they did not expect the police effort — 13 suspected gropers and flashers were arrested over 36 hours last month — to make a big dent in the problem. But, they added, it was a start.

"I feel better they caught these guys," said Juliette Fairley, 35, an actress who said that she encountered a flasher on her N train at 42nd Street not long ago. "But there will always be people out there like this."

Some crime and subway experts with long memories offered a cautionary tale yesterday. A subway police squad in 1983 and 1984 looking for lewd behavior led to the false arrest of scores of men, most of them black and Hispanic. The men were accused of "bumping," the jargon for men who rubbed up against women, and other petty crimes.

The arrests turned out to be part of a scheme by transit police officers to inflate their productivity and win promotion, and it became a major scandal. "It is extremely hard in a crowded subway station to tell right from wrong when somebody is up close to somebody else," Richard Emery, a lawyer who won a class-action suit on behalf of the falsely arrested men, said yesterday.