ABOARD THE RIO GRANDE ZEPHYR -- An impossibly blue sky arches over the Vista-Dome, the conductor stops by to point out scenic spots on the surging Colorado River below and the dining car menu offers boneless Rocky Mountain trout.

Travel takes a leisurely course on the nation's last privately operated intercity passenger railroad, an island of gracious 1940s living in a sea of budget-conscious transportation.

And so, of course, the Rio Grande Zephyr of the Denver & Rio Grande Western Railroad, connecting Denver and Salt Lake City, will soon be no more.

Gone will be the matched set of silver-gray passenger cars, the Vista-Dome views of the Rockies, the linen-covered tables and white-coated waiters, the steak breakfasts and fresh fish dinners. Amtrak is scheduled to engulf the Rio Grande Zephyr, completing its decade-long takeover of all America's long-distance railroad passenger service, at precisely 12:01 a.m. Monday.

From all over the United States, men and women have come to take one of the last possible rides on this 11-car piece of railroad history.

"This is the ultimate," said Robert Seavey, an engineer-designer from Weston, Mass. The Vista-Dome, a stainless steel streamliner, found on only one Amtrak line these days, "is the culmination of the car builder's art," said Michael L. Burshtin, rail vehicle project manager for New Jersey Transit.

Over the Continental Divide, along ridges of pine, spruce and fir patched with melting snow, through more than 30 pitch-black tunnels, meanders the old train, carrying 292 people.

Most of the passengers on this trip had hoped to take the entire 570-mile trip from Denver to Salt Lake City. But huge earth slides in Spanish Fork Canyon, 65 miles south of Salt Lake, buried the track and forced the Rio Grande Zephyr to cut its usual trip in half.

This is an old story for a rail line chiseled into steep cliffs and bored through thick mountains over the last century. Much of the work, said Denver & Rio Grande Western executive Leonard Bernstein, was done "under extremely primitive conditions," with teams of horses and sometimes unreliable explosives.

But today the passenger line, despite occasional rock slides and blizzards, maintains one of the best on-time records in the country, which in turn helps it keep its profitable freight cars running on schedule.

Once the track has been rebuilt, Amtrak will use the D&RGW line to take its new California Zephyr trains from Chicago to Seattle, San Francisco or Los Angeles. Amtrak then will abandon its less scenic Denver-to-Salt Lake City route through the Cheyenne area, Rock Springs and Green River, Wyo., leaving the state of Wyoming with no Amtrak service.

Tickets for the last Rio Grande Zephyrs over the Continental Divide were sold out quickly. Train crews worked all night to keep the aging engines in shape. If the 28-year-old diesel locomotive breaks down, parts are unavailable. "You have to make the parts," said freight train engineer Tom Derryberry, who is taking his wife, Karla, and their three children on a last ride.

Charles Raines, 42, a 727 pilot with Continental Airlines, flew his private plane from Los Angeles to Salt Lake City to catch one of the last eastbound Rio Grande Zephyrs. When he discovered the earth slides had cut the train off, he flew on to Grand Junction, Colo., taking the train to Denver and then back. "Trains are a lot more exciting than airplanes are," he said.

Train buffs like Don Ball Jr., stretching out of windows to take pictures for his Steamtown Rail Museum in Bellows Falls, Vt., could recite the names of the old lines combined into the D&RGW after World War II--the Denver, Northwestern & Pacific, from Granby to Orestod; the Denver & Salt Lake Western through the Dotsero Cutoff; the old, narrow-gauge Denver & Rio Grande from Pueblo through Glenwood Canyon; the Rio Grande Junction Railway from Newcastle to Grand Junction.

The airplane never killed the romance of rail travel, but it certainly killed its ridership and profits. By 1971 so many passenger lines were losing money that Congress created a quasi-governmental company, the National Railroad Passenger Corp., better known as Amtrak.

It quickly absorbed almost all the nation's profitless passenger lines and began a partially successful effort to stop the decline of ridership by cutting the system in half and investing in new equipment for the lines that survived.

Most private railroads were happy to give up their passenger service, but D&RGW held on. Along with the Southern Railway--which gave up its passenger service in 1979--the Colorado company feared Amtrak interference with its profitable freight traffic and said it believed its losses on passenger service were not yet great enough to merit paying Amtrak a fee to take over the service.

D&RGW maintains good service on its passenger line, and has kept a high sheen on the matched set of passenger cars bearing names like Silver Sky, Silver Aspen and Silver Pine. Amtrak will store the old cars, but may refit and use elsewhere some of the old Vista-Domes. Their second-deck passengers enjoy a 360-degree view the Amtrak Superliner cannot duplicate.

The Vista-Dome seats fill early on this run. Valleys, gorges, ghost towns and the bubbling headwaters of the Colorado pass by as the train eases up the steep grade at 25 mph.

In the dining car, so many passengers rush for a last taste of breakfast steak and eggs, Rio Grande Zephyr style, that steward John Hardin and his white-coated waiters must keep serving until noon. Amtrak would be hard pressed to provide the same spotless white tablecloths and menus with small prayers printed on them--"As you travel over this bountiful land of ours, may you be ever reminded of the grace Almighty God has bestowed upon us . . . ."

Hardin, 32, after genially dispensing travel information and seating passengers for four hours, betrays some unhappiness. Amtrak has its own service crew and will not need him and his men. "Some of these waiters have worked in the finest restaurants in Denver," he said.

The Amtrak takeover works out fine for other D&RGW employes. The quasi-governmental line will need engineers like Derryberry and dispatchers like Forest Van Schwartz, who will mostly handle the private railroad's coal-laden freight traffic anyway.

Although D&RGW has been losing $3 million a year on its passenger line, its freight has kept it in the black. When Van Schwartz, 39, began working for the railroad during college, he said, its stock was worth $7 or $8 a share. Now it sells for about $55.

That means little to John Denny, who has a stereo systems business in Denver, or to his wife, Sue, and their 4-year-old son, Ian. They sit in the Vista-Dome, recalling the blizzard blowing past the huge windows on one New Year's trip and the herd of 200 elk they saw during another.

Binnie Avery, 58, a Boulder housewife, says she has ridden trains all over the world, but thought she ought to get a last ride "on this one in my own back yard." Chuck Anderson, 38, an editor for Sunset magazine, says asking why people like trains "is like asking why you like sex. It's pretty hard to think of the reason, you just do."

The train shoots into the 6.2-mile Moffat Tunnel, 9,239 feet above sea level. Here it crosses the Continental Divide under James Peak. Donald Hulse, the brakeman-conductor on this trip, remembers the freight train the previous week that broke an air hose and was trapped in the middle of the tunnel for an hour. It was so deep, he said, even the company's special-band radio could not reach it.

Hulse's father and grandfather worked for the railroad. He plans to continue, serving on the coal trains, but his son, Rod, 15, wants to go to the Air Force Academy and be a pilot. Father and son can only share their model trains. "I may get out some of my old Lionels, the American Flyer," Hulse said.

After 10 minutes in the tunnel, a tiny bright star appears ahead. The train finally chugs back into daylight, now on the west side of the Rockies, at the Winter Park Ski Resort. D&RGW will continue to run a weekend ski train up the mountain, Van Schwartz said.

That is good news to people like Bill Henry, Marysue Wilcox, Krys Losasso, Connie Malick and Alan Mercer, who have abandoned their jobs at Dougal's Mountain Inn Restaurant in Grand Lake, Colo., for a last giddy ride on the old train to Glenwood Springs.

Their boss had returned from his last train ride glowing from the experience and dispatched his entire staff on a holiday. Glenwood Springs is a popular hot springs resort, not far from the ski village of Aspen, and a principal reason that Amtrak spokesman Clifford Black estimates this new line will add $3 million a year to Amtrak revenues.

Amtrak will accelerate to everyday service instead of the leisurely three days a week of the Rio Grande Zephyr. "Amtrak is not in the nostalgia business," said spokesman Black. "We are in the transportation business."