It was no surprise to discover that the Tokyo subway system is fast and efficient. What I was not prepared for was the scale. The stations are vast underground cities containing shops, cafes, bars, travel agencies and police stations. The 13 colour-coded subway lines intersect, run parallel and cross each other in bewildering confusion before unravelling towards distant suburban terminals. The newcomer needs to spend half a day deliberately getting lost just to figure out how to get around the world's biggest metro system, and the other half trying to find the exits from mega-stations like Shinjuku, through which more commuters pass daily than the number of people living on the island of Ireland.
It is clear that when the Aum Shinrikyo cult members released deadly sarin gas in the Tokyo subway on April 20th, 1995, killing 12 people and injuring 5,000, they were attacking the very heart of modern Japan.
With Tokyo so spread out and traffic jams common, the subway lines are the pulsing arteries of the city along which flows all classes of people, from office cleaners to corporate executives (whose chauffeur-driven limousines wait to carry them home from subway stops).
Among the intriguing statistics is this: the average subway rider in Tokyo spends three hours a day underground going to and from work, often in tightly packed compartments. Sometimes commuters have to be forced through carriage doors by white-gloved people-pushers. Many subway users learn to sleep standing up, held in place by the crush of bodies.
On late night trains the drivers give a little jolt of the brakes to wake up boozy office workers who have dozed off. For those who over-do it there are overnight cots at terminal stations. There are also shower rooms at the biggest stations to freshen up, and special shops selling souvenirs from different Japanese cities, so that an office worker having an affair can produce evidence to back up a story of an out-of-town business conference.
Unlike London or New York there are no buskers on the Japanese underground but some stations have recorded birdsong, and in others a burst of popular music will signal the arrival of a train. Everything is spotlessly clean. There is no litter, no graffiti. Theft and muggings are practically unknown.
What the newcomer finds really strange on the Tokyo subway is that men openly read pornography or thick comics with lurid sex scenes, displaying no embarrassment even when sitting beside schoolgirls in sailor uniforms and white socks.
The sexual antics of a few male Japanese commuters have, however, become something of a national scandal. Groping and rubbing against female passengers in crowded carriages is sometimes called "elbow sex" but it can be more intimate. Women have emerged from the crush to find their clothes stained with semen and sometimes even slit by razor blades.
For years this behaviour was treated as a nuisance rather than a crime. Until recently there was even a magazine for gropers called Finger Press which recommended the best subway lines and provided stills from groping videos.
A 1994 survey in Osaka found that nearly three-quarters of women interviewed had been groped on the metro, but only two per cent reported it to the police. Things are changing now as women begin to speak out. Arrests have become more common. Police at Ueno station detained more than 30 men this year, twice as many as last. Women officers in plainclothes have staged sting operations and recently caught 200 "finger-pressers" red-handed on a notorious line in one week.
Japan has another unique method of coping with the problem. Frustrated fondlers can go to one of Tokyo's "image clubs" and act out their groping fantasies in a simulated subway carriage, with prostitutes dressed as female commuters or schoolgirls. As a last resort, Tokyo is experimenting with women-only carriages, and have chosen the Chuo line as a test. The newcomer (male) to Tokyo must be careful to avoid getting into the first carriage of the Chuo line at rush hour as this is now reserved for women seeking protection from secretive groping.
The experiment is proving popular - though some women feel that they may be more vulnerable in these carriages - and the metropolitan police recommended last month that it should be more widely used. This means, however, that the Tokyo subway may soon have to add to its list of unusual statistics the fact that it is the only segregated underground system in the world.
The Irish Times will publish a supplement on Japan on Wednesday, December 10th.