The international airlines don't tell you -- they would prefer you didn't know -- but you can buy seats on many scheduled flights to destinations abroad at prices substantially less than what the airlines advertise.

The seats aren't sold directly to travelers by the airlines themselves, but by travel firms that, the firms say, have obtained bargain rates because of the large volume of business they do with the airlines. The firms pass at least part of the savings along to the customer.

The discount airline-ticket market is something you have to search out, and some travel agencies decline to participate in it. In effect, a two-tiered rate structure has developed. There are the official prices you learn about when you call the airlines; and there are the bargain deals some travel agents seem able and willing to obtain for travelers looking for the cheapest fare.

For example:

* Call the Korean Air Lines reservation number, and the agent will inform you that the lowest round-trip air fare between Washington and Hong Kong this month is $1,197. However, Embassy Travel, a Washington travel agency, recently offered a Hong Kong fare of $820 through the month of May -- and the flights are aboard Korean Air Lines.

* Embassy also offered around-trip fare to Tokyo of $1,050 on Northwest Orient Airlines (also only through May). The lowest 30-day advance-purchase fare available from the airline itself this month is $1,232.

* A limited number of Europe-bound passengers will be able fly a U.S. airline on scheduled service from Washington to London this summer for a round trip of $494, a fare available through the Bethesda Travel Center, a travel agency. The standard 21-day advance-purchase fare from Washington in May is $629 and from June through August it's $669 (weekday travel).

* First Class Travel, another Washington travel agency, is selling round-trip seats on Pan Am to the Caribbean island of St. Maarten for $313 through the summer. A Pan Am reservation clerk quotes the lowest fare at $466.

Similar savings -- from 15 to 40 percent -- are frequently available from these and other Washington travel agencies on regularly scheduled flights to much of Europe, Asia, Africa, South America, the Caribbean, Australia and New Zealand.

Often travel agencies will incorporate the lower fares in a tour package that includes hotel accommodations and sightseeing excursions, but some also sell only the air tickets.

A glance at recent Sunday travel ads shows such discount fares as Tokyo, $879; Bangkok, $969; Sydney, $1,150. Comparisons between official and discount fares are difficult, because of frequent seasonal price changes and other variables, but these fares are less than the lowest fares quoted recently by several airline ticket offices.

"We can save the clients a lot of money," says Kares Jhangiani of Wholearth, one of the Washington travel agencies regularly offering the low-cost fares.

These fares, it should be understood, are not for travel aboard charter flights, another kind of budget air travel. In fact, they are available mostly to destinations not normally served by charters from the United States, such as the Orient, Australia, South America and Africa.

Charters are special flights arranged by tour operators, who in effect rent a plane and try to fill it by offering seats at rates lower than the scheduled carriers. Charters tend to fly to Europe in the summer and the Caribbean in the winter, usually on a once-a-week basis. Seating can be cramped if the operator tries to squeeze as many seats aboard as possible.

The discounted fares discussed here are available on scheduled carriers, which may have daily departures, or at least several a week. The destinations can be almost anywhere in the world served by international airlines. Roomier seating and other normal airline services -- seat reservation, worldwide ticket offices -- are the same enjoyed by full-fare passengers.

The bargain tickets, however, almost always carry substantial restrictions (though restrictions on a charter may be even more severe). Also, only a limited number of seats on a flight may be available at the low price. And fares can change quickly, so you may have to pay when you make a reservation to take advantage of the price.

"The less expensive the ticket, the more restrictions there are," says Ray M. Greenly, director of consumer affairs for the American Society of Travel Agents. He advises inquiring travelers to ask about any conditions attached to bargain fares.

The most common restriction is that the ticket cannot be used on any other airline. If the carrier you are flying is delayed for any reason, for example, you can't switch to a competitor, as you could with a full-fare ticket. Additionally, a penalty of $50 or more probably will be imposed for canceling or changing the flight dates. The amount varies with the airline involved and the destination.

In some cases, travelers who miss the flight, or otherwise fail to use the ticket on the date for which it was issued, could lose the entire price they paid. Sometimes you won't know what airline you are flying until a few days before you depart.

And some travel agencies will accept only cash -- no credit cards -- in payment for the discount tickets. Others will accept charge cards for flights on certain airlines. (Whatever the price paid, the ticket issued is likely to carry the airline's official fare, not the discount fare.)

Despite these drawbacks, the discounted fares offer a big savings to travelers who have flexibility in their itinerary (should a flight be delayed) and who fully understand the restrictions.

The Bethesda Travel Center has posted a cartoon in its office that shows an airline hostess announcing to the passengers:

"Fasten your seatbelts, extinguish your cigarettes; and we'd rather you didn't discuss with each other how much you paid for this flight."

That pretty well sums up the international air-fare situation these days. As a result, it can pay to shop around.

Where are these cheaper fares coming from?

Because of strong competition -- particularly on routes to the Orient -- many wide-bodied jets frequently fly with empty seats. Unable to fill the planes at advertised rates -- established in part through international agreement -- some international airlines are unloading seats at a discount to travel firms.

According to travel firms selling the cheaper fares, the seats may be offered at what the industry calls a "bulk rate" -- under which the firm is obligated by contract to sell a minimum number of seats.

The bargains are best from fall through spring, when vacation travel is down. But travelers can find discounted rates even during the summer peak travel period.

Usually, the firms getting the good deals are wholesale travel companies, such as Travel Committee of Owings Mills, Md., and Travel Wholesalers International of Falls Church, who then make the lower fares available to retail travel agents. Occasionally, an airline makes the cheaper fares available directly to the travel agency.

Either way, at least some of the savings ultimately is passed on to the passenger.

"There's a real effort underway for all airlines to sell every last seat," says Clifton N. Cooke, copublisher of Jax Fax Travel Marketing Magazine, a monthly sold primarily to travel agents that carries up-to-date lists of hundreds of discounted and charter air fares.

"A seat sold is a seat sold," says Ruth Obre, sales coordinator in Korean Air Lines' Washington office.

Some airlines that sell at bulk-rate fares are very sensitive about publicizing the fact. The reason is that they don't want to lose the customers, especially business travelers, who pay the higher fares. A spokesman for Northwest, an airline that freely acknowledges selling "a certain given number of seats" to wholesale firms, points out, however, that the tickets "come with such restrictions that most people don't take them."

Some airlines are so touchy that they prohibit travel wholesalers and agents from telling the passengers what airline they are going to be flying until a few days before the departure. Nor is Jax Fax permitted to list the airline name in its compilation of discount fares.

Some wholesalers are worried that if word about discounts becomes widespread, the airlines will withdraw the low rates.

Cooke suggests that most of the big international airlines are involved in bulk-rate fares, and some admit it. "You can name any airline," he says. Pan Am acknowledges it sells seats to "large wholesalers," but will not elaborate. A Washington traveler, inquiring about a discounted fare to London, was told by a travel agent the flight would be aboard World Airways, flying from Baltimore-Washington International.

If somewhat secretive, the discount ticket market seems to be legitimate, although, says Ted Lopatkiewicz, a public affairs officer in the Department of Transportation, "I can't say we understand this whole industry yet." DOT was given responsibility for overseeing air travel regulations when the Civil Aeronautics Board was eliminated at the end of 1984.

The department currently is studying the market, in part to see if any of the bargain deals involve a violation of airline tariff regulations. "Frankly, we don't know if anything is wrong," says Lopatkiewicz, who points out that there is no prohibition against airlines selling seats to wholesalers at a bulk rate. Nor is a customer who buys a cut-rate ticket doing anything illegal.

But, says the DOT spokesman, "we have seen these ads, and they have peaked our curiosity."

The department's primary goal, however, is to determine if consumers are sufficiently protected in this volatile market. "The tickets obviously are a bargain," says the spokesman, "but with a bargain, there is a risk. Our concern is the impact on the consumers."

Unlike charter flights, which operate under a number of federal consumer protection laws, the discount market is not regulated. One problem with the budget fares is that customers may not be told, when they buy a ticket, about the restrictions it carries. Another is the possibility that an unscrupulous promoter of discount flights could take your money and disappear. The Transportation Department is not aware of such operators, but says they are a potential threat.

"We've had a lot of inquiries" regarding the cut-rate fares, says the spokesman, but thus far, "we've not heard of anybody with a worthless piece of paper."

So how do you find a discount fare?

* Consult an established travel agent with whom you are familiar. An agent cannot call up the discount fares on a computer, because they are not listed on the computer network. But many agents do subscribe to Jax Fax, which lists low-cost scheduled and charter fares. "Jax Fax is our encyclopedia for discounts," says the Bethesda Travel Center.

Be aware that the cheapest ticket may involve an indirect flight to your destination. You may want to pay more and arrive a half-day earlier. At one point last month, Embassy Travel was offering both an $820 and a $950 round trip to Hong Kong. The higher fare was aboard a direct flight to Hong Kong; for the lower fare, the traveler would be routed via Seoul, adding about six hours to the trip.

In some instances, a travel agent may not be able to confirm a reservation immediately. Since discounted seats are not listed on a computer, the agent may have to call the wholesaler, who may have to check with the airline itself. A confirmation may take two or three days.

For a comparison, make sure you know the lowest fare the airlines are charging to your destination. You don't want a heavily restricted budget ticket if the savings aren't worth it.

* Some local travel agencies -- such as Embassy Travel, Wholearth, Trans Am Travel, Amerasia and Landmark -- frequently advertise discount fares in this Travel section. Some ads will say specifically "scheduled" airlines. If the type of service is not included, the fares could be aboard either scheduled or charter carriers.

Make sure you know exactly what you are buying:

-- Ask if the flight is scheduled or charter.

-- Make sure you know about any restrictions. What happens if you miss the plane? Will it cost you to change the reservation on any leg of the trip?

-- Try to find out as soon as possible which airline you will be flying. Then check with the airline directly to confirm your reservation.

-- Make sure the routing is acceptable.

Remember, too, that travel agencies selling the lower-priced tickets often specialize in certain markets. So the agency who gets you a good deal to the Orient may not offer the best buy to Africa. Sometimes the embassy or national tourism office of the country where you are flying will tell you where to try for the cheapest ticket.

STEAM EXCURSIONS: Several special steam train excursions through the Virginia countryside have been scheduled for this month and next, departing from the Alexandria station. They are sponsored by local rail fan clubs.

* Skyline Limited: The train travels via Manassas to Front Royal at the northern tip of Shenandoah National Park. Time for lunch in Front Royal before the return trip. May 18.

* Piedmont Limited: Via Manassas to Charlottesville, with time for dinner and sightseeing. May 19 and June 1 and June 2.

* Grand Circle Steam Tour of Western Virginia: A two-day trip. Via the Shenandoah Valley and the Appalachian foothills to Roanoke. May 25-26.

Ticket prices vary according to destination and type of rail car. For more information: Steam Train, P.O. Box 456, Laurel, Md. 20707; (703) 569-0366 or (301) 292-6481.