WASHINGTON is strewn with great dreams that somehow went awry. But few of them are quite as stunning as the Union Station-National Visitor Center - Amtrak terminal disaster. That white elephant sits ponderously on the side of Capitol Hill - incomplete, underused, deteriorating. About $46 million has already been spent on a project that was supposed to cost $16 million. Much, much more than another $46 million would be required to complete the great master plan. In fact, close to another $46 million, give or take a few, would be needed even to finish a sharply curtailed version of the plan. And the price tags keep going up every year. It is time for Congress to decide what the future of that project is to be. In our view, that means facing up to the mistakes of the past and being willing to cut losses rather than send more good money after bad.

The money that has been spent so far - $16 million by the owners of Union Station and the rest by the federal government - has produced one major accomplishment. Daniel Burnham's magnificent building has been preserved. But that, we regret to report, is the end of the good news. The Visitor Center is too big and too empty. The new Amtrak terminal is too small and too inconvenient. The projected parking garage has shrunk in size and quadrupled, at least, in cost. The highway system that was to feed the garage and bring the visitors is incomplete. And one of the rationales on which the whole project rested - the need of the city for a place to put Bicentennial tourists - disappeared 16 months ago.

It makes no sense to try to assess blame for this mess in any precise way; there is enough of it to go around - twice. The original plan, which we supported vigorously, underestimated the construction and financing problems. The execution of that plan was, in retrospect, hopeless. Inflation, labor problems, cost overruns, inadequate supervision, a law suit and a bankruptcy all helped turn a dream into a nightmare. What remains to be done is a salvage operation.

The proposal made to Congress by Secretary of Transportation Brock Adams and Secretary of Interior Cecil D.Andrus is a starting point. They have recommended that $52 million in federal funds be spent primarily to: 1) repair the existing structures, 2) move the train station back into the old Union Station building, 3) complete a 1,400-car parking garage (the original plan called for a 4,000-car garage) and 4) make some roadway improvements and acquire certain property interests at the station.

No one can argue, we think, with the need to make repairs. The roof, electric system and heating system of the old building are in bad shape. Certain other structural support and fire-protection measures are also needed. Altogether, the bill for those repairs is estimated to run close to $15 million.

The other major two Adams-Andrus proposals are controversial. They recommend spending about $12 million to move the train station, tear down the part of the new station that has been already built, and put the railroad tracks back where they were to start with. That isn't so outrageous as it sounds. It would cost $9.5 million to finish up the new station so it can handle the passengers that Amtrak is expected to generate. And a new station would mean two buildings to operate and maintain instead of one. It would also mean that the Park Service would have the vast acreage of Union Station for a Visitor Center that it says can be handled well in less than half the space. We think the two secretaries are right, even though it means giving up the dream of a great transportation center that would bring together trains, buses, subways and helicopters.

The garage is another story, and it is there that Congress should consider cutting the losses. The estimated cost of finishing that building is $13 million. Its 1,400 parking spaces would be intended for the use of visitors and railroad passengers. But it seems to us almost inevitable that the spaces would end up being used, as are practically all the spaces in that part of town, by people who work on Capitol Hill. In addition, the area's new multibillion dollar subway system serves Union Station, well, and the emphasis ought to be on getting both tourists and train passengers out of their cars and onto the subway. So the best thing to do with the existing skeleton of that garage may be to protect it against the weather and let it sit there. Maybe someday someone will think of something that can usefully and economically be done with it. In the meantime, it would serve as a monument to broken dreams and badly laid plans - a monument not wholly out of place in a nation's capital.