But, for all that it was designed by clever engineers in Paris, the Metro here has been totally Mexicanised, or rather has been transformed into a mixture of what the country is today and what most Mexicans would like it to be. So, the system is high-tech because Mexicans don't want to be seen as backsliders on the way to the new millennium. Its stations are clean and their marble pavings and walls are burnished and polished as though to banish the vision of a land where many still live without water and sanitation. Some trains have Muzak because Muzak is modern. As in Paris, many stations display art and culture because millions of Mexicans aspire to a better understanding of these things.
And ancient Mexico won't be banished. Pino Surez station, near the city's main square, has been carefully built around the lovingly protected remains of the main temple of the Aztecs, whom the Spaniards conquered when they arrived here in 1517.
In a country where millions still can't read, each station has a symbol as well as a written name. The Airport station has its little aircraft symbol, as you would expect. But some are more sophisticated: the Montezuma station has a representation of the Aztec emperor's feathered head-dress; the Zapata station has a symbol of the massive hat of the famous revolutionary Emiliano Zapata; the sign for Etiopa station is a lion's head, symbol of the emperors of Ethiopia, lions of Judah. And so on. It is almost impossible to get lost on the network.
In recent years a change has come over the Metro. Financial crises have hit the poorest Mexicans and today there are beggars and hustlers on the lines who were never to be seen when the system first opened. As in above- ground Mexico City, they come in all sorts: small children selling sweets and cheap fountain pens; blind beggars singing solo above the hum of the train or going from carriage to carriage with guitars and portable electronic keyboards and often travelling in pairs.
On the line to the university the other day, I saw a fierce young man with steel-rimmed glasses and a Trotsky beard shouting the merits of his paper whose headlines were extremely derogatory to the government. He reminded me of the Mexican left, which has a proud, but not always effective, place in Mexican politics. Perhaps it was a coincidence but we had just passed through Coyoacn station, which serves the house, now turned into a museum, where the Russian revolutionary lived in exile until he was killed with an ice-pick on Stalin's orders in 1940.
Ten minutes away, at the Hidalgo station on the same line, religious Mexico is powerfully present in the Metro. Some months ago devout Mexicans saw a likeness of the Virgin of Guadalupe, the country's patroness, appearing in the concrete in the tunnel. That piece of concrete has been quarried from the wall by courtesy of the Metro management and is now on display on the pavement above ground at one of the city's busiest crossroads. All day, a quiet crowd of worshippers line up at a makeshift shrine to kiss the foot-high image of La Guadalupana, who first appeared to a peasant on a hill to the north of the city in 1531. Godless Trotsky isn't allowed to have it all his own way, you know.
Meanwhile, the management makes its own genuflection to decorum. Between 6pm and 9pm in the busiest stations some passageways are reserved for women, thereby freeing them from the groping hands of Mexican males.
Despite the fact that uniformed and plain-clothes police are constantly on patrol, one sad manifestation of present Mexico occasionally comes to the Metro: armed robbery. If you are unlucky, someone will stick a knife in your ribs and ask you quietly for your money. The locals say that it's much better to yield to them gracefully. It tends to avoid unpleasantness.
Middle-class Mexicans are horrified if a visitor does decide to take the Metro, and forecast terrible things if he or she does it again. "But," says Ron Buchanan, a local editor, "there's probably a good dose of snobbery in their thinking. The middle class don't like being with poor people. They want a public transport system of their own."