It's summer 1987. Brilliant sunlight pours through the skylighted roof of a renovated Union Station, bathing the historic terminal's marble floors, granite columns and gilded arches in a soft glow and breathing new life into Washington's long-decaying monument to the American railroad era.

By day, the vast edifice bustles with travelers, tourists and local citizens -- some dashing for trains and others strolling leisurely through an array of shops, cafes and flower stalls. By night, patrons dine at elegant restaurants or take in movies downstairs at six theaters.

This vision -- a mixture of turn-of-the-century magnificence and contemporary commercialism -- is embodied in a series of newly completed architectural plans for the colossal depot that are scheduled to have their first public airing this week.

The plans, drawn up since last summer, call for massive changes in the station's interior, ranging from constructing new mezzanines for shops and restaurants to restoring ornate Pompeiian wall stenciling and aging clocks. Passengers would have shorter distances to walk to trains. The sweeping colonnaded exterior would be preserved.

Officials say that the transformation of the now-vacant structure will cost nearly $140 million in federal and private funds, but that the rejuvenated terminal will pay its own way and no longer depend on federal subsidies.

"When the sun's shining, this is really a great space," said Keith Kelly, executive director-president of the Union Station Redevelopment Corp., the federally backed nonprofit group set up to oversee the renovation, as he stood in the cavernous main hall of the dilapidated building. "You can see there's lots to be done yet."

The plans were drawn up by Benjamin Thompson & Associates, the Cambridge, Mass., architectural firm that designed Harborplace in Baltimore, Quincy Market in Boston and Waterside in Norfolk. Some critics who have examined the plans have charged that commercialism appears predominant, overshadowing the building's classic contours and uncluttered spaciousness.

The design relegates the building to "a mere shell into which commercial space will be inserted," complained the federal Advisory Council on Historic Preservation in a recent letter, contending that the overall plans pose "serious problems." The council also cautioned against using "marketing and retailing formulas which may have proven successful in shopping malls," but may be inappropriate for the station.

According to the plan's advocates, the proposed retail outlets are intended as a key source of revenue for the redevelopment enterprise. And they contend the shops are designed to blend into the majestic setting.

These issues are to be debated at hearings and other review sessions in the next few months, and officials said some revisions are possible before work starts next summer. Several changes already have been made, including eliminating some proposed trees and fountains criticized as cluttering the interior.

The plans are to be presented tomorrow at a meeting of the District's Historic Preservation Review Board and will later be considered by other agencies, including the National Capital Planning Commission and the Commission of Fine Arts.

The architectural plans are being unveiled at a time when the Reagan administration is proposing to end federal subsidies for Amtrak, raising new uncertainties about the station's principal rail service. Without federal aid, Amtrak officials have warned, the railroad would go out of business.

Congress, however, is considered likely to balk at the administration's proposal. Federal officials contend that Amtrak might operate even without the subsidies. And officials overseeing the station's renovation say their plans for commercial development would remain sound, even without the railroad.

"Union Station is going forward very well," said Secretary of Transportation Elizabeth Hanford Dole, rejecting suggestions that the proposed Amtrak cutoff might jeopardize the redevelopment project.

The proposed renovation will be the second major overhaul for the station, which was opened in 1907. In 1976 the federal government converted it into the National Visitor Center, moving the tracks, building a new terminal and constructing a slide show pit in the main waiting room.

By 1981, when it was ordered closed to the public because of leaks in its roof and other deterioration, the government already had invested $117 million in the station, stirring widespread controversy and criticism. The building has remained closed since then, and railroad passengers have had to walk around the three-block-long structure to the train terminal erected behind it.

Two years ago, Dole launched the redevelopment effort, relying partly on $70 million supplied by Amtrak. Around $25 million more is expected to be provided by a development team, including Equity Associates Inc., an affiliate of Chicago-based LaSalle Partners Inc., and the Baltimore-based firm of Williams Jackson Cavanaugh Associates.

In addition, work now is under way to complete a five-tier parking garage behind the station, which was left unfinished in 1976 because of multimillion-dollar cost overruns. The District plans to spend up to $40 million in federal funds for the 1,300-space garage and other projects.

Today, the deserted Union Station complex appears water-stained and crumbling. In one second-floor office, large mushrooms are growing on a damp floor. Paint flakes from ceilings. Fixtures are missing. Carpets and walls are soiled. Only preliminary repairs have been carried out.

From these modern ruins, architects plan to resurrect an ancient Roman scheme, drawing on such classical structures as the Baths of Caracalla dating from 212 A.D. These relics of Imperial Rome, officials note, had inspired Chicago architect Daniel H. Burnham's original design for Union Station.

The Roman baths were "a city within a city," said Philip Loheed, a vice president of Benjamin Thompson & Associates. The baths have served as a "prototypical model" for the renovation project.

"What we want you to do is walk in and say, 'This is a grand public place,' " Loheed added. The building, he said, would be a transportation hub, tourist site and center for dining, shopping and entertainment. "There is a lot of retail in it, but that is not the dominant thing."

Planners say they also hope to recreate the station's long-forgotten turn-of-the-century decor. Analysts have scraped off microscopic paint samples to uncover original pigments and have pored over buried floors, hidden walls, old fixtures and deteriorating statues and columns.

"It will be a very dramatic revelation that what used to be done can still be done today," said Paul Childs, a vice president of Harry Weese & Associates, an architectural firm preparing for the restoration work. "We can begin to appreciate the richness of some of the styles of the past."

New white and red marble floors are to be laid, officials said. Ceilings are to be trimmed with gold leaf. Beige damask wall coverings with floral patterns and Pompeiian wall stenciling in gold, red and green may be restored. Dark red scagliola, or artificial marble, is to be repaired.

Under the overall plans, the basement, now used partly as a parking lot for Amtrak employes, would be converted into a decorative setting with fountains, curving stairways, public tables for snacks and six movie theaters.

Officials said the proposed theaters would offset a shortage of movie houses in the downtown area. The theaters also would have convenient access to the Metro subway system's stop at Union Station.

On the ground floor, a vaulted entranceway would lead to ticket counters and gates for rail passengers.

The eastern and western wings and a long concourse to the north would be filled with shops, restaurants, newsstands, tourist booths and other outlets. The station's ornate Presidential Suite, which is at the eastern end, has been suggested as a site for a gourmet restaurant.

An extended mezzanine, overlooking the main halls, has been planned for shops and cafes. New offices would be built in the building's upper stories, an additional revenue source. "There has to be a mix -- the historic with the commercial," said Kelly.