"In the beginning you feel scared, and you feel guilty along with the pleasure. But gradually you discover the amazing thing - that no one stops you. When I was younger, it was a kind of sexual outlet, but these days it's more of a psychological game - the thrill of touching a woman I don't know in public, watching her, and taking pleasure in her response."
Samu Yamamoto, a small, unremarkable-looking man of 44, is the most famous pervert in Japan. A few years ago his autobiography, Diary of a Pervert, sold 50,000 copies before it was withdrawn by the publishers under pressure from women's groups. He produced a sequel, Confessions of a Pervert, and these days makes a living as a producer of specialist pornographic videos, all set on commuter trains. As a celebrity chikan, or groper, he is an experienced interviewee, and parts of his narrative strain credibility (he insists, for instance, that he met his wife when he groped her on a train). It would be comforting to write him off as a tasteless self-publicist, but the only unique thing about Mr Yamamoto is the freedom with which he talks about his hobby.
At home and abroad, Japan is renowned for the safety of its streets. By the standards of the developed world, and even assuming that some of it goes unreported, levels of violent crime are low - in a population of 126 million, complaints of sexual assault, including rape, come to little more than 5,000 a year. But this enviable state of affairs ends at the doors of the country's trains.
Tales of groping have long been part of the folklore of commuting and every other woman seems to have a story about the activities of the chikan. For years, the problem was nervously laughed off as little more than a distasteful nuisance, but recently things have been changing: Japan has begun to recognise both the extent and the human cost of low- level sexual crime.
Nobody knows if the problem is really getting worse, but the signs are Japan is undergoing something of a groping boom. Film-makers like Mr Yamamoto have established molesting as a distinct sub-genre of Japan's massive 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 last month, chikan even had their own magazine, Finger Press, which contained stills from groping videos, groper comic strips, confessional letters from gropers and even reviews of the best spots for groping ("On Shiki station, there are a lot of girls in very short miniskirts," runs one notice. "They have exhibitionist tendencies, but they look a bit unpredictable.") The current edition of Finger Press, however, will be the last: at pounds 7.50 an issue it was selling a mere 30,000 copies, instead of the anticipated 40,000.
For the victims there is, of course, nothing funny about it at all. Three years ago, a survey by women's groups in Osaka found that 71 per cent of women canvassed had been groped and that for many the effects were lasting. Victims found themselves too nervous to travel by train, and fearful of crowds. One student stopped going to university after being molested morning after morning, and feelings of shame and self-disgust were encouraged by the fact that the most successful groping is so subtle as to be completely deniable.
In the past, Japan's police did little to tackle the problem, although that has changed with a concerted campaign in Tokyo. The capital's two biggest stations, Tokyo Central and Shinjuku, have permanent chikan counselling corners, staffed by female officers. For one week earlier this year 70 officers were mobilised on the Saikyo Line, the city's most notorious, where half of all reported groping incidents take place. Thirty of them patrolled the carriages in plain clothes, and bagged an average of six people a day. Of these a third were charged, and the rest were released after their misbehaviour had been reported to their families, schools or employers.
"There are two types of chikan," says Yumi Kakisako, a female police sergeant who works at the counselling corner in Shinjuku station. "Some of them do it half by stupid accident - when they're caught once, the shame is so great that they'll never do it again. But then there are those who get a real pleasure from it, the compulsive chikan, and they are harder to cure: we see the same faces again and again." The Metropolitan Police Agency is compiling figures on the number and nature of chikan offences for use in future campaigns.
Preliminary results suggest chikan range from schoolboys to retired men and appear to have nothing in common in terms of social background or employment status. Their victims are equally diverse: although most are between 15 and 25, the attraction they hold for their assailants seems to have little to do with what they wear or the way they look.
The biggest unanswered question is why epidemic groping should be such a peculiarly Japanese problem. Rush hour trains in the cities certainly do become claustrophobically crowded, but not so much more so than in other countries. Samu Yamamoto puts it down to "the crazy attitude which Japanese men have to sex" - and the ambiguous, covert nature of commuter groping has its advantages in a society which is often content to ignore what it cannot see, and where the pressure to maintain outward harmony often makes it easier to suffer in silence than speak out.
Indeed, the biggest factor may be the reluctance of Japanese women to make a fuss. "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 Sergeant Kakisako. "There's a tendency among these men to convince themselves that if a woman doesn't say anything, she's enjoying it. The best thing the victims can do for themselves is to speak up."