Groping has become such a problem on Tokyo's commuter trains that the police are pressing for women-only carriages. The scale of the nuisance contrasts oddly with Japan's otherwise low crime rate. Richard Lloyd Parry on the secretive world of the commuter pest and his victim.

Japan's trains, as any visitor to the country quickly discovers, are unique: the cleanest, most punctual and most efficient in the world. The poshest of the new bullet trains include conference rooms, nappy-changing facilities and digital screens carrying the latest share prices. Between Tokyo and Osaka, after check-in times have been taken into account, it is quicker to travel by rail than to fly.

But at the humbler end of the transportation scale, commuter travel is remarkable for a very different reason: Japanese trains are home to the most highly organised, obsessive and persistent gropers in the world.

Yesterday, in Tokyo, the Metropolitan Police announced the latest initiative in an ongoing effort: Japanese railway companies are being urged to provide women-only carriages in an attempt to stamp out a near-epidemic of aggressive fondling.

Tales of the chikan, or gropers, are part of Japanese urban folklore, and most women who have commuted on busy subway lines for any length of time have their stories. Three years ago, a survey in Osaka found that nearly three-quarters of women interviewed had been groped at one time or other, many of them repeatedly. For there is more to the problem than just the opportunistic quick feel which occasionally afflicts women on crowded trains in Britain. Groping in Japan is an institution and a fetish, with a core of dedicated practitioners and an even larger crowd of armchair aficionados.

The activities of the chikan take a wide range of unsavoury forms. At its most basic, they are so subtle as to be undetectable: a suspicious pressure on the legs from an unidentifiable hand in a mass of compressed commuters. But the worst kind of chikan takes advantage of the packed state of rush-hour trains for much bolder and more invasive groping. Stories are common of women alighting from a packed train to find their clothes speckled with semen, or even slashed open for better access by a surreptitiously wielded razor blade.

Recently, there has been something of a groping boom. Yesterday it was announced that arrests of chikan at Ueno, one of Tokyo's biggest stations, had doubled since the same period last year. Films featuring chikan heroes have become an established sub-genre of Japan's huge porn industry. In Tokyo's "image clubs" - themed brothels, with mocked-up classrooms, changing rooms or oriental harems - there are simulated train interiors where customers can act out their groping fantasies for pounds 100 an hour, with prostitutes dressed as office workers.

Until recently, chikan even had their own magazine, Finger Press, which contained stills from groping videos, groping comic strips and even reviews of the best groping venues. When Finger Press closed down, after "disappointing" sales of a mere 30,000 a month, its editors were besieged by pleading phone calls from tearful "armchair" chikan.

For years, the problem was treated as a little more than an embarrassing nuisance. But this is giving way to recognition of both the scale of the problem, and its human cost. The idea of women-only compartments is just the latest in a series of anti-groping initiatives taken by the Tokyo police. Permanent chikan counselling centres, staffed by female officers, have been set up in Tokyo's biggest stations and sting operations have been carried out, featuring policewomen in plain clothes. In a week-long undercover operation a team of 30officers bagged 30 chikan a day on the Saikyo Line, Tokyo's most notorious, where half of all offences are committed.

On almost every other count, Japan has one of the lowest crime rates in the world. Why should groping be epidemic in a country where as many murders occur in the space of a year as take place in America in one day? Crowded trains have something to do with it, but they are not in themselves uniquely Japanese. Perhaps the biggest reason is the failure of the victims themselves to speak out, and the reluctance of bystanders to intervene.

"Compared to other sex offences, I think that women as well as men don't have much of a notion that this is a crime," says Yumi Kakisako, a female police sergeant who works with chikan victims.

"The best thing the victims can do for themselves is to say `No!'"