After nearly four decades of ferrying the harried and the hurried across the taxiways of Washington Dulles International Airport, those lumbering beasts known as mobile lounges may be rumbling into retirement.

The regional airport authority is scheduled this week to approve the layout for a new subway to connect the Dulles passenger terminals, a massive undertaking that could ultimately cost nearly $900 million and end an era once touted as the future of aviation.

The aging shuttles were the brainchild of Eero Saarinen, the internationally acclaimed architect who designed Dulles' signature swooshed-roof main terminal, and made this sleepy suburban outpost in 1962 the first American airport where travelers were transported to and from their planes.

"When I first got on one, I thought it was a 1950s attempt to be in the future, like a space program," said Richard Hannay, riding a mobile lounge one recent afternoon from the main terminal to the midfield concourse as he prepared to fly home to London.

"It's almost like something from 'Star Wars,' " said Carol Giangiordano, of Tampa, after her 16-year-old son marveled at these odd, white creatures roaming the tarmac as their plane touched down.

These hoary beasts, roving on massive tires that have the look of mastodon hide, come in two kinds. One, call it the male of the species, sprouts a pair of large white horns atop the roof. These horns contain the machinery that allows the shuttle to shift up and down so that it can connect to both airplanes and terminal buildings. The other, serving only terminals, is flat up top.

But now, with Dulles securing its position as the region's busiest airport, any archaic sci-fi fantasies have been dispelled by the sheer number of travelers. The lounges are little better than slow, boxy buses, hard-pressed to handle more than 12,000 travelers an hour.

That's hardly enough to keep pace with today's passenger loads, much less those projected for early next century. The airport expects to handle 20 million passengers this year, and 55 million within a couple of decades.

Travelers, eyes ever on their wristwatches, have become less enchanted than exasperated.

"They're a pain in the neck," said Debbie Hull, riding a lounge to her Florida-bound plane. "You hyperventilate while you stand and wait."

And as the number of flights has soared, the shuttles must increasingly yield for airplanes on the taxiways, further frustrating time-pressed passengers. The shuttles "got the job done day in and day out. But now they're old and they're tired," said James A. Wilding, president of the Metropolitan Washington Airports Authority.

It's the question that airport officials say they hear most often from passengers: When are you going to get rid of the mobile lounges?

Authority directors are expected to take a major step in that direction tomorrow when they cap a half-year of deliberation and endorse a design for a three-mile airport subway loop. The authority could approve the initial funding this fall, with design and engineering to start as early as next year, according to Wilding. It would take at least five years before the first segment opens.

Although the airlines serving Dulles support the development of a subway, they remain skeptical of the proposed layout, suggesting that a single line straight between the buildings, rather than a loop, could handle the loads while shaving $270 million off the cost, according to the authority's projections.

But airport planners say the loop system would be more reliable, convenient and--when fully complete in about 25 years--better able to accommodate peak crowds. While the final price tag is estimated at $898 million, the initial segment linking the main terminal and the midfield concourse would cost $182 million, airport officials said.

The system would operate 18 trains, similar to those now in use at the Atlanta and Denver airports, with half running clockwise and half counterclockwise along a pair of tracks. Running at least once every 90 seconds, the trains would carry more than 35,000 passengers an hour, planners projected. The subway line would intersect each of the three midfield concourses ultimately planned for Dulles at two points.

The mobile lounges date from a time before the airport ever built its first midfield concourse and once conveyed travelers directly to their planes. They were initially dubbed mobile lounges because, airport officials boasted at the time, they would indeed be traveling waiting rooms on stilts and wheels, where pampered passengers could smoke and listen to music in "complete comfort," coddled by the latest technology in air conditioning, protected from weather and jet blasts while attended to by stewardesses.

But with the passing of the years, the stewardesses were reassigned, the cozy reading lamps removed and many of the seats yanked out so that more travelers could be crammed in. Subway-style metal poles and straps were installed, leaving passengers mobile, but hardly lounging.

"It's such a hassle. It's such a long wait to get out of here, and people keep piling on. It gets so crowded before they pull out," said Bruce Shackelford, of Fairfax, wedged into the corner of the shuttle, his briefcase crammed by his feet against a sign beseeching travelers to keep the area free of bags. Around him, the odor of sweat swelled with impatience.

As the tempo of travel has accelerated in recent years, passengers are increasingly ill-tempered while waiting: first at the ticket counter, next at security, and then for the mobile lounge. For arriving travelers, weary from their flight, the final leg of the trip by mobile lounge can be the most aggravating.

"I finally get in and I'm thinking I get to go home. Then I suddenly realize, oh no, I have to wait again to get on a mobile lounge," said returning businessman Will Kastroll, whose wife and three children were waiting for him at their Herndon home as he languished in the lounge. "They're awful. It's the worst system in the whole world."

CAPTION: Officials at Dulles International Airport are planning to replace the crowded mobile lounges with a three-mile subway system connecting the terminals.

CAPTION: The mobile lounges made Dulles the first American airport where travelers were transported to and from their planes.