TRAVEL INSIDER : Air-Rail Combinations Save Time and Money : Transportation: Travelers can fly United one way, take Amtrak the other and receive discounts on both.

The idea sounds great: We’ll take the train for a change. We’ll save money, enjoy the scenery and arrive at our destination happy, relaxed and untainted by airports. Then someone calls for schedule information, and it becomes clear that there just isn’t time to make the whole trip by rail. After all, anything more than a four-day weekend these days is viewed not as a vacation, but a sabbatical.

But there is a compromise: Take the train one way and then fly home.

In most quarters, this idea will be immediately shouted down by seasoned travelers, who have learned that airlines and Amtrak do to the wallets of one-way travelers what Hoovers do to stray lint. When I raised the idea of an air-rail combination to one veteran travel agent last week, in fact, she fairly shouted back over the phone.

“ Not! “ she said. “One-way tickets cost an arm and two legs.�

Most travelers, and many travel agents, don’t realize that for the last year and a half, Amtrak has been quietly building its Air-Rail Travel Plan, with United Airlines as its partner. The program was unveiled Jan. 1, 1991, after several years of no nationwide air-rail programs at all, and was substantially expanded and simplified this year. In some cases, the Amtrak-United package can undercut independently booked one-way fares by as much as 25%.

The United-Amtrak program breaks the nation into four zones, setting one price for north-south itineraries along a single coast, two other prices for journeys to the mountain states or the Midwest, and another price bracket--the highest--for transcontinental trips. The air portion counts toward the United frequent flier mileage totals.

Travelers should recognize that the program is built on coach fares, and reserving a train sleeper compartment adds hundreds of dollars to the bill. On the other hand, up to three stops are allowed on the train portion of the trip. On the plane portion, connections are allowed, but no overnight stops unless the airline schedule requires it. The whole itinerary must be completed within 45 days, and current prices will stand through Dec. 31.

A few examples:

The Transcontinental Travel Plan. You fly nonstop from Los Angeles to New York. From New York, you ride the train south to Washington, D.C., and stop over for a night or two. From there, you continue by train to Chicago, stop over again; then to Lamy, N.M., which is a 17-mile shuttle-bus ride from Santa Fe. After that stopover, you continue by train to Union Station in Los Angeles. The price: $529 during peak season from May 22 through Sept. 13 and Dec. 12 through Jan. 3 of next year; $449 from Sept. 14 through Dec. 11. (Without the program, a traveler this month would pay at least $329 for the one-way train fare and $380 or more for the one-way flight.)

The Midwest-West Travel Plan. You take the train from Los Angeles to Seattle, then stop over. A day or two later, you continue by train to Glacier Park Station in Glacier National Park in Montana and stop over again before continuing by train to Chicago. From there you fly back to Los Angeles. Peak price: $449; off-peak, $389.

The Mountain States-West Travel Plan. You fly San Diego to Denver. When you’re ready, you take the train west through the Rockies (and the 40-odd mountain tunnels that begin soon after the train pulls out of the Denver station) to Salt Lake City. From there, you continue by train to Las Vegas and stop over. Then take the train back to San Diego. Peak price: $389; off-peak, $329.

The West Coast Travel Plan. You take the train north from Orange County to Oakland, and board a free Amtrak bus into San Francisco, where you stop over. Boarding the train again in Oakland, you ride straight through to Portland, Ore., where you either stop over or keep on going to Seattle. From Seattle, you fly back to Orange County. Peak price: $399, off-peak, $349.

About sleepers: On a Los Angeles-to-Chicago train, an economy-class sleeper compartment (a 4-foot-6-by-6-foot-6 space with two seats that convert into upper-lower sleeping berths) with three hot meals daily costs $295 beyond the original train fare. (The price is tied to the space, not the number of people occupying it.) For a deluxe sleeper (a 7-foot-5 by 6-foot-6 space that sleeps up to three and includes private bathroom and hand-held shower; meals included), the price is $639 beyond the original fare.

East of Chicago, the options multiply to include sleepers without meals and sleepers with toilets but without showers. And on all trains, demand is high. On the July 13 trains from Los Angeles to Chicago, for instance, 41 of 42 sleeper compartments were reserved. (Amtrak operators at 800-USA-RAIL can offer more information on sleeper fares.)

The rail-air idea was born, United and Amtrak officials recall, when Amtrak approached the airline four or five years ago. Next, the railroad tested the rail-air link idea with a handful of short-term promotions, which served selected areas. The prospects for a national program remained bleak, however, until MTI Vacations, an Amtrak contractor and purveyor of package tours, devised a software system that could marry the reservation systems of Amtrak and United.

The first-year target for Amtrak’s Air-Rail Travel Plan program, Amtrak spokeswoman Pat Kelly recalls, was 15,000 bookings. Despite minimal advance promotion, the offer drew 18,000 bookings. Some 40,000 are expected this year.

“It was an idea that made sense,� said Dave Herren, United’s director of leisure sales. “We were really receptive to it here . . . It’s not going to be a huge market, but it’s fun stuff.�

It is not, however, United’s highest priority. Promotion of the program has been principally Amtrak’s responsibility. And with the railroad’s limited budget (ticket revenues covered 79% of Amtrak’s operating costs last year; federal subsidies covered the rest), the program’s public profile remains relatively low. Posters have gone out to travel agents and scattered print ads have appeared, but in terms of national exposure, acknowledged spokeswoman Kelly, the program’s exposure is “quite limited . . . We haven’t advertised it on television at all, ever.�

I tried a cold call on another travel agent--Suzanne Hofsommer, a consultant with Infinity Travel in Seattle--and she had heard of the program. But she said she hadn’t seen much of anything about it since it opened.

“It’s just not something most people are aware of,� said Hofsommer.

There’s no getting around the time a train takes. The Denver-San Diego route described above, for instance, clocks in 31 hours, 50 minutes with no stopovers. But if you’re ready for that pace, the Amtrak-United program could be an attractive alternative. More information, including details on packages that also include hotel rooms and sightseeing tours, is available through Amtrak’s “Great American Vacation� line at (800) 321-8684. (Don’t call United; clerks there don’t have computer access to the air-rail reservation system, and may know nothing about it.)

The United-Amtrak air-rail possibilities are likely to grow even more intriguing in April, 1993, when Amtrak will start running the nation’s first transfer-free transcontinental trains from Los Angeles to Miami (estimated traveling time: 58 hours).

Assuming the airline-rail program remains in place (United officials say the contract for next year isn’t signed yet), serious rail buffs could take the transcontinental train, savor the idea of being among its first passengers, celebrate by eating too much in a Cuban restaurant, then fly home from Miami. For those more inclined to music than Miami, there is the prospect of riding the rails to the ever-more-popular New Orleans Jazz and Heritage Festival (annually held in late April and early May) and flying back.

Amtrak hasn’t scheduled April, 1993, yet, but if current departure times continue, travelers could board a train at Union Station in Los Angeles late on the evening of a Sunday, Tuesday or Friday, and get off in New Orleans not quite 48 hours later. Or they could start sooner and include stopovers of two nights or more in Phoenix or San Antonio or Houston along the way. A transcontinental train, Southwestern vistas, jazz and New Orleans. Hmmm.