Of all the major historic buildings in Washington, none still standing has been subjected to more foolish abuse than Union Station, the well-nigh imperial monument to the romance of rail travel situated so happily in relation to the Capitol and many of the city's other principal attractions.

There was, of course, the ignominious "pit," that carpeted burrow in the floor of the Main Hall fashioned to accommodate a monumentally silly slide show for those few befuddled visitors who happened by the place when it was supposed to be the city's main Visitor Center. Then the building was simply closed, to be subjected inside and out to predictably nasty damages resulting from its vast, porous roof and countless leaky gutters, while passengers were (and still are) forced to sneak around the back to catch trains.

For these reasons, especially, it is necessary to accentuate the positive when reviewing designs that were unveiled this week as a prelude to putting Union Station back into presentable, and useful, shape. In accordance with a federal law adopted in 1981, these designs guarantee not only the saving of the great old place but also its restoration. If the plans work out, the Union Station we will see when it reopens in 1987 will be in crucial respects the same impressive structure its first users saw 77 years ago.

But in other respects, also crucial, it will be a vastly changed place. The strategy of that 1981 law was to promote private commercial development inside Union Station in order to ensure its lasting economic health. If this makes practical and even theoretical sense, as I believe it does, it also presents a tremendous design problem, which, simply put, is how to alter the place dramatically while respecting its essential spirit.

Whether the architects -- Benjamin Thompson & Associates of Boston for the commercial development and Harry Weese & Associates of Chicago and Washington for the restoration and train passenger service improvements -- have satisfactorily solved this problem remains very much in question.

In brief summary, this is what is proposed: The principal facades and interior spaces are to be restored according to the highest standards of the craft. The unpleasant Amtrak station behind the grand old edifice will be demolished. A new "link structure," already under construction, will serve rail passengers and connect the old building to a new elevated parking garage (upon which construction finally has recommenced). New ticketing, baggage and information facilities will be inserted into the interior of the old building. And, as the proposal now stands, 191,000 square feet of restaurant and retail space will provide the economic engine that makes the whole thing go.

But the designs submitted Wednesday to the city's Historic Preservation Review Board were by no means complete. Although certain issues, particularly those involving location of major elements, were aptly addressed, key questions remain partially or totally unanswered. Where, precisely, will the added elements be placed in the Main Hall? How big will they be? What will they look like? What, indeed, will any of the additions, including all of the stores, look like? Will they fit in with or contrast to the elements of classical style Daniel Burnham so effectively employed in his original design? How will they affect the interconnected flow of majestic spaces and the variety of natural light sources that are among the chief glories of the building as it used to be?

Because of questions like these, I share many of the worries and reservations expressed by the staff and members of the review board. Indeed, it would be folly not to worry some at this point. Though today but a shell of its former self, Union Station remains one of the greatest achievements of the era of great train stations, in sheer architectural quality not far off the standard set by the late, lamented Pennsylvania Station in New York, which was demolished 22 years ago. This clearly is a building that deserves our profound concern and the very best efforts that our architects, engineers and restoration specialists can give.

The restoration work presents no problems other than those of technique and cost that may come up. ("If we find gold-leaf trim underneath the paint up there on the ceilings, it'll be gold leaf," says Keith Kelly, president of the Union Station Redevelopment Corp., "although obviously if it comes out to be $20 million worth we'd have to reconsider.") The demise of the so-called train station now in use can only cause cheers. The "link structure" is a bare-bones affair with an exposed steel truss ceiling, handsomely designed by Sverdrup & Parcel Associates of St. Louis and Washington.

The new Amtrak service counters are to be centrally placed along the rear wall of the concourse, which is the long, spacious, sky-lighted room that, in pre-Visitor Center days, served as the connection between the Main Hall and the train platforms. This seems just right, for it preserves the clear progression of spaces that distinguished Burnham's design. More than this can't be said, because the counters -- which at 200 feet long are indeed a major element -- have not yet been designed. Architect Paul Childs of Harry Weese & Associates says they will be "sort of contemporary in style with classical features compatible with the historic building," which is sort of encouraging.

Far less so are the floor plans and perspective renderings submitted by the Thompson firm. The new floor plans are, I hasten to say, a significant improvement on those exhibited last summer, when the results of the competition for rights to develop Union Station were announced. The architects proposed, in the earlier versions, a large, Piranesian cut in the floor of the Main Hall, which someone dubbed, not unfairly, "Son of 'Pit.' " In the revised plan, this floor cut, necessary to provide access to basement-level retail facilities, is provided in the concourse structure.

But the plans have entirely too crowded a look to them: too many trees too idiosyncratically placed, narrow corridors separating the concourse stores as if freedom of movement inside the spacious room is something to be frowned upon, and, perhaps, too many stores altogether. Colden Florance, an architect and member of the review board, was right to worry aloud about long, low ceilings and the oppressive, constricted feeling they can produce.

He was right, too, to urge the architects "to push in the direction of the Beaux-Arts image" when designing details such as storefronts, free-standing pavilions, railings, balconies and graphics. Although the Thompson firm did not provide any detailed studies of what these elements might look like, the loose perspective renderings it did exhibit, with their shopping-mall-like, Anywhere-USA lines, were sufficient to give pause.

Though not without fault, the losing designs submitted to the competition by the Washington office of Skidmore, Owings and Merrill possessed a crystalline sense of classical detailing that really seemed to fit the Burnham building. It is of course possible to create an attractive design in these spaces with contrasting rather than similar elements, but if the Thompson office continues in this direction, it will have to do better.

Still, Jane Thompson, vice president of Benjamin Thompson & Associates, was right in her feeling that the review board was fundamentally supportive. "These are matters of immense study," she said. "When a final aggreement is reached between the non-profit USRC and the private development team, still negotiating details between them , then we will be authorized to spend our time doing the final and detailed design. I think you just have to hope and assume we are enormously sensitive to these issues. We're not doing anything assertive, Post-Modern, cute or tricky. We're going to be greatly restrained and refined, without aping what is there."

More power to them. In the meantime, one has to be powerfully reassured by Washington's unique architectural review processes. In a case as complex and as important as the adaptive use of Union Station, members of agencies such as the city's preservation board, or the National Capital Planning Commission, or the Commission of Fine Arts, can make immense contributions.