Paris? London? What's the difference, when you can hop a train after work as if it were the Metro and be in one or the other in time for dinner?

This autumn, the superfast Eurostar celebrates its 10th year, darting between the twin capitals in a scant two-and-a-half hours. Eclipsing distance--not to mention centuries of history--Eurostar has made Paris a virtual suburb of London. As for London, it's the hip new arrondissement northwest of Paris.

At least, that's Eurostar's take on the Chunnel "commute." With a cheap new 35 euro Paris-to-London fare--less than a taxi to Heathrow airport--it's marketing the cities as neighborhoods. Touristic cliches like Big Ben and Piccadilly are out. Instead, Parisians are invited to pop over for the Notting Hill Festival or go clubbing at the likes of China White, a Soho nightspot so chic with young Parisians that half the staff are French. To Londoners, Eurostar pushes Paris Plage, the uber-cool Seine-side beach.

Call it tunnel fusion. The casual blending is reflected in any number of ways. Much has been made of the bloodless British conquest of the French countryside, with untold numbers of Brits buying old French farmhouses and driving up real-estate prices. Yet while English retirees and second-home buyers may have ditched Devon for Dordogne, the younger set has different ideas. The British community in Paris has doubled since 1997 to 70,000; meanwhile, 262,000 French live in London--the second-largest foreign-born group in the city after the South Asian communities. And in both places, it's well-educated twentysomethings driving the traffic.

Julien Audran, 25, is one of them. A Parisian working for Citigroup in London, he makes the trip every weekend to see his French fiancee. "It's possible to live in two cities," says Audran, who hops the Eurostar Friday evenings and returns Monday aboard the 6:30 a.m. train for London. "It's full of young French bankers on their way to work."

Paris, meanwhile, is acquiring a similarly distinctive British cast. In May, English-speaking expats launched Paris Live Radio, the first English-language station in Paris. And far from a refuge for Brits craving BBC World Service comfort alongside imported Marmite, Paris Live aims to help Brits live in Paris like Parisians might. Located on a leafy alley near the Moulin Rouge, it serves up news only locals can use: weather, sports, theater and concert announcements. "English-speaking people want to feel they're part of the town," says the station's director, Richard Booth.

Passenger traffic is up 19 percent this year alone, and Eurostar believes this is only the beginning. Who knows what cultural transformations might follow? As a Liverpudlian at The Bombardier, a British pub on the Place du Pantheon, puts it: "The French are coming around to our ways. They're drinking pints!"