FOR A LONG time our British cousins have held a big edge over our brilliant American technology in mass transportation with their good train and excellent bus and subway service, and now they have stepped out in front once again by extending London's fine Underground right into the middle of London's Heathrow Airport.

Once you walk out of Customs control in Heathrow now, the clearly visible signs (a very British trait) point the ways to automobile parking, taxis, buses and trains. And the Heathrow Underground trains run on the Picadilly line, which means with no change of trains or lines you ride right into the heart of London - to Knightsbridge in 35 minutes, Picadilly Circus in another three or four minutes.

This Heathrow Central Underground station is, indeed, something of a miracle for the international tourist who travels light, with baggage he or she can carry or wheel along comfortably. Now that tourist does not have to wait for a motorcoach to fill up and wind its way to the mid-London airline terminals, then flag a taxi or a bus or catch the Underground to their hotels.

True, the distances from the three terminal buildings are long, as they are in most big airports, but there are long, fast and smooth-moving sidewalks most of the way. Unlike most American airport operators, the British Airports Authority provides supermarket-style luggage carts free, and permits the public to take them from the three terminals right up to the turnstile entrance to the Underground, and to wheel them onto the moving sidewalks for easier transit. This is important because from Terminal Three, the international flights building, to the subways is about a quarter of a mile; from Terminal Two, handling most European flights, to the subway is about 600 feet.

Strangely, the opening of the extension to Heathrow Central last December has not been widely publicized to American tourists through the travel trade, but London Transport Authority, which operates the Underground, British Airports Authority and British Airways, which operates the coaches between Heathrow and its two mid-London terminals, are sponsoring publicity campaigns at home and abroad. Underground officials estimate that one quarter of all Heathrow air passengers are using the subway trains, which have already achieved 75 per cent of the forecasted traffic volume.

Because Heathrow has a curfew on jet plane flights, none arriving or departing through the night after 11 p.m., the fact that London's Underground shuts down between midnight and 5:30 a.m. does not hamper air travelers. First train leaves Heathrow Central weekdays at 5:07 a.m., 6:48 Sunday; last train leaves 10 minutes before midnight. During morning and evening peak rush hours there are 15 trains an hour, one every four minutes; 12 trains an hour in the middle of the day, eight trains an hour during the evening.

There will be a London-wide fare increases on all transport systems today, and the Underground ticket from Heathrow Central to Knightsbridge will rise to 90 pence ($1.75), to Picadilly 1 pound ($1.88).

The motorcoach fare to and from mid-London air terminals, now 1 pound, probably also will go up because British Airways is delighted that the traffic on its airport coaches has dropped sharply because of the subway connection. The airline had to maintain frequent coach service to get air travelers for B.A. and 23 other airlines to and from Heathrow, but this was and is an unprofitable adjunct to its airline and airport operations. Where British Airways was dispatching buses from its city terminals every six minutes to Terminals One and Two, it now sends out only one bus stopping at both terminals, and thus hopes to reduce its operating loss. The bus trip takes about 45 minutes, depending upon traffic.

From my Knightsbridge hotel to London West Terminal the trip took seven minutes by taxi and cost $2.10. In light mid-morning traffic my coach made it to Terminal Two in 28 minutes, for a total of 44 minutes from hotel to Heathrow at a cost of $4.10. My time coming into London from the airport via Underground had been only a couple of minutes less, cost less than half as much, and required no changing from motorcoach to taxi.

The French have a slightly more complicated rapid transit connection between Charles de Gaulle Airport and downtown Paris. This still shiny, new airport works great when passengers are embarking or disembarking and riding the undulating moving sidewalks from the plane gates down to the terminal's center, but then it gets a little confusing. Direction signs are more American than British in that they can be more confusing than clarifying.

Buses are supposed to leave the terminal every 15 minutes for a spanking new railroad terminal. I climbed aboard at 2:50 and the small bus was jammed with standees when it pulled away at 3:05 for the five-minute run to the railhead. A train - a clean, new, well-lighted, comfortable railroad commuter car - arrived a couple of minutes later, and in 24 high-speed minutes we were in Paris at Gare du Nord. The combination bus-train ticket cost 10 francs (about $2), and the Metro ticket for the subway from that station to I'Opera, 1 franc 70 centimes (about 35 cents).

Returning to the airport by subway and motorcoach from Porte Maillot, where the new Convention Center now stands, was $2.60 for the coach and 35 cents for the Metro. Because it was in the 5 o'clock traffic jam and because of a couple of minor accidents on the expressways, it took more than an hour for the 12-mile trip. On July 1, all transportation rates in Paris are expected to rise by about 15 per cent.

Because of its two extra model changes - bus to train and train to subway - the French airport connection is less convenient than the British, but it is still better than the hassle passengers go through to get to and from most major American gateway airports. Better in that both are faster, easier, more comfortable and cost less.

Which raises the question, long asked and not yet properly answered: Why can't America's major airports be connected to their metropolitan and capital cities by rapid transit systems that work?

Such projects are not, of course, inexpensive. To make Heathrow the world's first major international airport linked directly to the capital city's subway system cost 30 million pounds (nearly $60 million). This cost was split between the British government (25 per cent), the Greater London Council (25 per cent) and London Transport, which operates all London buses and subways (50 per cent).

London Transport expects 12 million passengers to use the airport extension every year. The shuttle bus that ran between Heathrow and the previous end of the Underground at Hounslow West, was discontinued when the airport terminal was opened, and airport employes and airplane crews now use the subway traveling to and from their homes. One British Airways supersonic Concorde captain I know flits across the Atlantic Ocean at 1,350 miles an hour, then rides comfortably home on the Underground. His co-captain explained (regretfully, it seemed) that he lives in the opposite direction, so must either drive or take a bus home after work.

It does seem strange that the interconnection between rapid ground transit and rapid air transit has been so slow in arriving. No other major gateways, beyond Paris and London, have achieved it.

Gatwick's link to London is a regular railway, and it is a secondary airport. Brussels has a nice, comfortable and convenient trolley car kind of rail connection that whips passengers quickly between Brussels' international airport and the Central Railroad Station in the heart of downtown Brussels. Tokyo's old airport has an immature monorail that runs from inside that airport only part way into Tokyo. Where it stops short, passengers must make a not-very-easy transfer to ground-level taxicabs or subway for the rest of the trip into the city. Tokyo's new airport, Narita, is reportedly at least two hours from downtown.

Cleveland, Ohio, has a trolley car rapid transit connection from its airport to the center of downtown, but Cleveland is neither a capital city nor an international gateway - although it does bill itself as the only U.S. city with a rail connection from its airport to downtown. If and when Oakland, Calif., gets the new airline services the Civil Aeronautics Board is proposing, its passengers may use the San Francisco Bay Area Rapid Transit system.

There is little evidence that Washington's new Metro will stretch out to Dulles International Airport in our lifetime, and although the subway does reach to Washington's National Airport, that field is not an international gateway, and at present it can accommodate only small jet planes. The Port of New York, New Jersey Authority and New York's Metropolitan Transportation Authority have been making almost inaudible noises for years about extending the Long Island Rail Road into Kennedy Airport. But this project, too, seems now, with inflation and fiscal crises all around it, to be even less than dormant.

The best that American airport authorities can do is to take to the radio on heavily traveled weekends during the peak travel seasons to warn air travelers to leave home an hour or two earlier than usual to allow for airport highway traffic congestion. As I was driving home recently after a quick trip across the Atlantic to London and Paris, using the supersonic Concorde and the London and Paris quick subway connections, this advice struck me as being only wryly amusing.