Now is the time to ride Amtrak's long-distance trains. The future of America's beleaguered passenger service is at best uncertain. Although it appears the Reagan administration will not succeed in its plans to eliminate the rail system, cuts in Amtrak service are likely. So catch it while you can.

From the overnight convenience of the Northeast Corridor's Night Owl to the ever-wondrous Southwest Chief, Amtrak offers a relaxed alternative to the crush of air travel. Among the best of its routes are:

* The Night Owl's New York Executive. How can you make an early morning New York meeting, avoid getting up at 5:30 for the first shuttle and spend less than half of what going up the night before would cost in transportation and hotels? Use Amtrak's New York Executive service on the Washington-Boston Night Owl.

The Executive is Amtrak's best-kept secret. For $77 ($126 round trip) you can have your own sleeping compartment between D.C. and the Big Apple, leaving Washington at 10:30, an hour and a half after the last shuttle. The Night Owl stops at Penn Station in New York at 2:44 a.m., but Executive service puts your car on a siding at the station; you can sleep until 7:30 and then walk leisurely to breakfast.

The Washington Executive -- the southbound train -- offers similar service. You can board your sleeper starting at 9 p.m., the train leaves New York at 3:52 a.m. and arrives in Washington at 8:30. A bottle of very drinkable wine and cheese are included, as are coffee and juice in the morning. The sleeping car attendants are generally on the friendly side.

The Night Owl goes to Boston, pulling in at 8:05 in the morning, and again offers an economical alternative ($115 one way, $164 round trip) to flying and booking a hotel the day before to make early-morning meetings. And for light sleepers or early risers, the train offers a sunrise over Long Island beaches. Or you can do a plane-train combination to Boston by catching the northbound Night Owl (11 p.m.) at BWI airport's Amtrak station and flying back to BWI the next day.

Sleeping on Amtrak is about the same as train sleeping has always been -- occasionally a bit tricky. There are noises in the night that you don't have at home, or in a hotel room. The loose thingamajigs that flap and buzz irregularly. And the toilet hidden under the bed. But you can get a reasonable night's sleep on Amtrak, particularly on the New York (and Washington) Executive run, where half of it is in a stationery bed.

And traveling by sleeper is a literal living history trip. It's the way generations before us moved across the country.

* The Crescent. The fresh flowers and tablecloths and dome car are gone, but Amtrak's version of the old Southern Crescent still gives America's most historic train ride. Leaving Washington daily at 6:30 p.m. and New Orleans at 7:30 a.m., the Crescent makes a 24-hour run between the nation's capital and the land of jazz.

It takes you through the old South -- where Sherman plundered and where antebellum structures still stand tall. Through miles of pine trees and azaleas and dogwood and live oaks and Spanish moss. Past old Studebakers, frame cottages with populated front porches and lots of red clay roads. By Howard's True Value Hardware, Bethel Church, Roy and Lynn's Deli, BINGO TONIGHT, Baber Rhyne Drugs, Starvin' Marvin Sells for Less, and the Piedmont News Stand. Through Tuscaloosa, Toccoa and Slidell, Birmingham, Anniston and Picayune.

Vintage Crescent is a four-hour stretch between Anniston, Ala., and Atlanta. Here the big train tortuously picks its way through ravines and hollows and hamlets, the engineer clinging to its throaty whistle like a lifeline -- which it really is for the old cars and trucks chancing the dozens of unguarded blind crossings in the Crescent's path.

At Anniston the stationmaster goes through a daily ritual that has transcended decades. Though we may be in the computer age, messages are still relayed to the train's cab on a giant sling shot propped up so the engineer can grab the slip of paper folded in the middle of the sling's cord as the train comes into the station. On a recent day one elderly woman and one train buff author awaited the Crescent as it rumbled into town. The engineer snared the paper and the Crescent's two passengers climbed aboard.

At about the same hour the Crescent is leaving sleepy Anniston -- but 2,000 miles away -- the southbound rolling beer party known as the Coast Starlight is treating passengers to a 113-mile stretch of Pacific Ocean vista. And 800 miles closer a crowd of California-bound Amtrakers are stretching their legs and buying jewelry from Pueblo Indians as the Southwest Chief makes a 20-minute stop in Albuquerque.

Amtrak's western trains offer quite a different experience than their counterparts east of the Mississippi. Their vistas are bolder, their passengers are bubblier, and their equipment is better.

* The Southwest Chief is Amtrak's second longest run (the first is the California Zephyr). Mile per mile (2,242 of them in all) it is Amtrak's best. The Chief's first 24 hours take you from the flatlands of the Midwest to the awesome deserts and mountains. The next 18 take you through cactus-studded Arizona, the Mojave Desert and on to the Southern California coast.

A vivid lesson in U.S. topography unfolds during the first morning of the Chief's westbound run. The train leaves Chicago at 3:10 p.m., and the next morning as you sit at early breakfast in the upper-level diner you can watch the land get higher and drier. The wheat fields of eastern Kansas give way to the butted and parched grasslands past Dodge City, and later in the morning are transformed into the desert of southern Colorado, with snow-capped Pike's Peak in the distance.

Trinidad, Colo., rolls into view at 10. Set against the Sangre de Cristo Mountains, this old town presents a movie-set western main street. The aging Columbian Hotel, now closed for renovation, once put up actor Tom Mix and his horse. Trinidad is a fun stopover -- regular train tickets allow stopovers at no extra cost -- if you enjoy authentic cowboy culture. Get off at its ancient depot (maybe, as happened with me, a cowpoke will give you a ride to the town center in his pickup) and take a room at one of the local motels (they range from the Holiday Inn and Best Western to the mom-and-pop Downtown Motel).

Stroll along some of Trinidad's brick streets (Trinidad brick was at one time among the best quality red brick available) down to Tano's cafe. Complete with swinging doors and a very portly gentleman named Tano, it offers some of the cheapest good meals anywhere. Have a rental car brought "over the mountain" from Raton, N.M., so you can explore the surrounding territory. (I phoned from my motel, then left word that I could be found at Tano's. During breakfast, two men in cowboy boots pushed open the swinging doors, one called out my name, and we signed our rental agreement over coffee.)

Monument Lake Park, about an hour west of Trinidad, is a very visual picnic and fishing stop. At night try one of Trinidad's eateries and sit in one of the old-fashioned saloons on its main street. If you are there on a Saturday you will probably see the police clear the street at closing time to keep down the Saturday night hoopla.

Raton Pass begins just outside Trinidad, and it's impressive either by highway or train. The Southwest Chief needs an extra engine tacked onto it to make it over the 7,834-foot pass, which marks the highest point on the Chief's journey and includes a mile-long tunnel. It is one of the highest points on any interstate highway (I-25, which parallels the tracks through the pass, except for the tunnel).

Raton Pass is filled with graceful rock formations and vegetation. Ruins of the once spectacular Baca ranch are nestled in the pass. Beyond the crest of the pass, just before the Colorado/New Mexico border, the train -- and road -- coast down into the strip town of Raton. If you drive to Raton to return your rental vehicle, you can add to your train adventure by reboarding at Raton's grand old adobe station. As you stand on the platform and 11 a.m. nears, the desert begins to rumble. An air horn sounds and 500 tons of Southwest Chief screech into town. Then it is five hours to Albuquerque through prime Northern New Mexico mountain and desert terrain.

The pageant that once was the Albuquerque stop is no more. The dozens of brightly dressed Indians waiting beneath the graceful arcades between the station and the Alvorado Hotel are gone, as are the arcades and hotel themselves. But a few Indians still bring their wares down for the 20-minute stop in this sprawling city in the Rio Grande Valley.

The Albuquerque-Los Angeles portion of the Chief run is a recommended sampler for those with limited time or tolerance for extended rail odysseys. Leaving Albuquerque at 4:15 p.m., it arrives at Los Angeles' beautiful mission-style terminal at 7:40 the next morning.

The roomy, full-length domed lounge car and the upper-level dining car are superb spots from which to view a sunset over the New Mexico desert. As with the rest of the Amtrak system, the meals served in the diner are not always exciting, but they are palatable and not exorbitantly priced. Service is adequate; furnishings are on the spartan side. But all tables are by the window and the view is unbeatable.

For the Albuquerque-Los Angeles overnight trip, you can rather comfortably get by with the Superliner coach instead of a sleeper. Copying the old Santa Fe "El Capitan" cars (which still run on some of the trains and are actually superior -- among other things the old cars have spacious restrooms, while the new Superliner cars have airplane-style closet-sized ones), these upper-level coaches include seats that recline far back and leg boards that can be folded out to make the kind of "bed" found in ultra-first-class overseas airplane accommodations. Pillows come with the seat and a car attendant is on duty. Unlike the sleeping cars, however, attendants in coach seem to have quite variable moods; the last time I was on this train the car attendant was decidedly on the grumpy side.

The L.A.-bound Southwest Chief provides a constantly moving picture: young Indians crowded in front of the Crazy Horse Bar in Gallup watching the train; an old woman holding up a board of jewelry and waving; desert jackrabbits running like, well, scared rabbits as the train moves across the desert; the old brick station at Flagstaff, Ariz. (elevation 6,900 feet and always cool at night); the mammoth and long-since-outmoded palm-tree-flanked terminal at Needles, Calif. (all summer, the hottest spot in the United States); and sunrise over the Mojave.

Then before 8 a.m. the Chief slides into Union Station, across from Los Angeles' famed Olivera Street. The station itself resembles a mission, with rounded arcades and quiet gardens. Olivera, the oldest street in the city, is a little bit of old, Spanish Los Angeles. The rest of the town is far in spirit from Union Station and Olivera.

* The Coast Starlight. This is Amtrak's most popular long-distance train and also its most fun ride. It leaves Los Angeles every morning at 9:55 (but the inbound Sunset Limited, for which it waits, frequently delays its departure), stops at Oakland (for San Francisco) at 8 in the evening and ends in Seattle 22 hours later.

The lounge car on the Starlight is inevitably crowded, with the drinks -- soft as well as hard -- flowing and the conversations lively. There is a lot to talk about on the Starlight, including a superlative 2 1/2-hour ride on the beach, some of which cannot be reached by paved road. The long-closed wooden station of Surf, Calif. -- where surfers once came by train to ply the waters of this relatively undeveloped area -- is reminiscent of the days when car travel in the state was limited. (At Honda, south of Surf, seven destroyers ran aground in 1923, and scores of sailors were injured. Trains had to be used to get them to hospitals. Just north of Surf the hull of a capsized Liberty Ship still pokes through the waves.)

The Starlight leaves the rocky beaches before San Luis Obispo and climbs slowly through the Santa Lucia Range. A maze of tunnels and hairpin curves provides frequent vistas in this usually dry part of the world. Down below is the Highway 101 freeway, with its steady stream of cars moving infinitely faster than the Starlight.

Salinas is at 5:30. The town that was "East of Eden," Salinas also is the station for the Monterey Peninsula and Big Sur. For San Francisco you can get off at San Jose (6:57) and take a Southern Pacific commuter train into the city, passing through the populous peninsula, or you can exit at Oakland (8:52), where an Amtrak bus picks you up to take you into downtown San Francisco.

The Starlight's ride north is in darkness until about Oregon, then through the very lovely Willamette Valley, into Portland (3:15 p.m.) and its handsome station, and past more vistas of distant mountains as it arrives at 6 p.m. in Seattle.

* Other Amtrak Long Runs. The California Zephyr, the Empire Builder and the Sunset Limited all have their own unique attractions.

Besides its spectacular climb across the Sierras into California, the Zephyr has a very exhilarating run on the route of the old Rio Grande Zephyr, a 10-hour, 370-mile run between Salt Lake City and Denver. Rarely dipping below a mile in altitude on this stretch, the Zephyr passes through picture-postcard desert and mountain terrain, including Ruby Canyon (which is truly red) in Utah and Glenwood Canyon in Colorado. The Moffat Tunnel, the longest rail tunnel in the United States, is on this route.

The eastbound Zephyr's Salt Lake-Denver run is particularly unusual in its descent into Denver. As it snakes down the mountain, you have the feeling of being in a very slow, long airplane as the lights of Denver's sprawl laid out far below gradually get nearer.

The Sunset Limited, 30 hours between New Orleans and Los Angeles, includes a whopping 15 hours across Texas. The West Texas portion of the Limited passes through some of the most remote territory in the country, with the most populous stop being Alpine, a town of 5,500 people between San Antonio and El Paso. If you liked the scenery in the epic movie "Giant," you'll love what you see from the windows of the Limited.

The Empire Builder -- Chicago to Seattle, 2,206 miles, 42 1/2 hours -- is a longtime Amtrak and pre-Amtrak favorite. Like the Southwest Chief, it starts in the midwestern flatlands but turns north through the rolling farms and forests of Wisconsin, through Minnesota and eastern North Dakota in the dark and then into the desert-y northern plains of North Dakota and Montana. Glacier Park (a summer stop only) is on the route of the Empire Builder, roughly two-thirds of the way between Chicago and Seattle. The Empire Builder is Amtrak's best bet for snow-capped mountain vistas.