The train station is on its way back to Washington's Union Station.

The move, which is subject to congressional approval, is part of a compromise plan worked out this week by officials who have been squabbling for years about each step in a grandiose project to turn the building into a National Visitor Center.

The latest plan to salvage what has been described as a monumental white elephant also calls for completion of a 1,400-space parking garage that now stands rusting and partially built at the rear of the complex.

The plan will cost $62 million, on top of $44 million already spent, to finish a job first authorized by Congress in 1963.

The new proposal includes razing the new train station, which has been described as looking like a motel lobby, and moving ticket counters, baggage and passenger waiting room areas and concessions back into the main Union Station building. The plan also calls for extension of the train tracks toward the station to reduce what now is a one-third of a mile walk from the front door, repair of the badly deteriorating main building at a cost of $15 million and completion of the parking garage and ramps to it.

If the work is completed, renovation of the classic Roman structure that began 21 years ago with an offer by the railroads to give the terminal to the government free of charge, will have cost $106 million.

Last October, legislation to complete the project at about the same cost - but to leave the train station where it is now - was lost in the last-minute rush by Congress to adjourn in time for the November elections. The measure had been held up because of a disagreement between the House and the Senate about the cost and scope of the project.

The key to the latest compromise was agreement by Rep. Harrold T. (Biz) Johnson (D-Calif.), chairman of the House Public Works and Transportation Compmittee, to allow the train station to be returned to the old main concourse.

The front half of the building will continue to be called the National Visitor Center, although the question of what to do about the expensive multiscreen slide show that is located in a pit just inside the main entrance is not addressed in the legislation.

The pit, known in bureaucratese as the Primary Audio Visual Experience (PAVE), was to have been the highlight of a Bicentennial exhibit that boosters predicted would attract millions of visitors annually.

But the crowds never came, and last fall, in an economy move, the slide show was stopped, and the staff and space allotted to the visitor center was reduced by about one-third.

The relocated train station will occupy 40,000 square feet, or about one-half of the concourse, which is the rear half of the two vaulted, cavernous rooms that make up Union Station.

The concourse now is only partly used. The west end has a restaurant and a walkway to the existing train terminal. The east half, which is carpeted and elevated on plywood, was used for displays during the Bicentennial but now is boarded off to conserve heat and light costs.

Work on the project was halted three years ago this month when the George Hyman Construction Co., which had won the contract to build the parking garage, announced that construction costs would run 19 percent, or $4.7 million, above its bid. Then Interior Secretary Thomas S. Kleppe ordered the work to stop to avoid further cost overruns.

Chairman Johnson said yesterday that "the next time we are going to let contracts for finished jobs - no more cost - plus contracts."

A spokeswoman for the Department of Transportation said that the train station will not be moved until the parking garage is completed.

She said that if Congress approves the legislation by this fall, work could begin next spring and would be completed in 3 to 3 1/2 years.

Chairman Johnson was more optimistic. He said work could begin later this year, an might be finished in 2 1/2 years.

Another refinement in this year's legislation will allow tour buses to use the bus deck on the parking garage. That portion of the garage is completed, but its use has been limited to Metro buses.

Johnson predicted that removing the restriction will encourage charter buses to load and unload tourists there, providing a steady stream of customers for the now underused visitor center. Johnson said he also hopes to work out a plan to require tour and charter buses to park somewhere north of the complex. Many of those buses now park on streets along the Mall and around Capitol Hill, marring the view, Johnson said.

The completed parking garage would reverse 1,200 of its 1,400 spaces for visitors, by regulating the time that cars can enter and leave the facility.

During the time when the parking garage was under construction, 200 parking spaces outside of Union Station, which is located just north of the Capitol, were eliminated to make room for a ramp to the garage.

When Amtrak threatened to sue becasue there was no place for train passengers to park, the city installed 70 metered spaces around the Columbus statue in the front of the station building.

The lack of parking in the Capitol Hill are: is a source of constant complaints from tourists.

Yesterday, the driver of a green station wagon looked pleadingly to Capitol Policeman J.T. Martin and asked, "Can you help a poor tourist from Pennsylvania find a place to park?"

Martin, who has been directing traffic at the east front of the Capitol for six years, pointed in the direction of Union Station and told the harried traveler, "go three blocks across the grounds to the Visitor Center."

As the car drove off, Martin admitted that "I don't know how many spaces are over there, but there's none here."

Committee chairman Johnson predicted that completion of the project would make the facility the focus of tourist activity, as he first envisioned in 1968, when as a member of the Public Works Committee, he boosted the project. At that time, the total cost was estimated at $16 million.

Over the years, as plans changed and inflation increased, the cost soared. One of th enewer considerations was the necessity to repair the turn-of-the-century building, designed by architect Daniel Burnham after the Roman Arch of Constantine and the Baths of Diocletian.

The transformation of the building from a train station to a visitor center was proposed at a time that railroad passenger service was disappearing. But in recent years, that trend has been reversed.

"This is a better plan" than the one considered last year, Johnson said yesterday. He said it has the approval of "everyone down," including the White House, Office of Management and Budget and the departments of Interior and Transportation.

The legislation was introduced Thursday by Rep. Elliott H. Levitas (D-Ga.). It asks for $39 million in new funds, to go along with $21 million previously approved. Antoher $23 million is available from the northeast rail corridor funds allocated by DOT.

Levitas, chairman of the House public buildings and grounds subcommittee, has scheduled a hearing Tuesday on the bill. Johnson predicted that the legislation could be approved by the full Public Works Committee by the end of next week.

Although the Levitas' bill essentially would accomplish what the Carter administration wants, it divides the responsibility for maintaining the complex between the departments of Interior and Transportation.

Robert Mendelsohn, special assistant to Interior Secretary Cecil D. Andrus, said the administration would prefer that the entire building be under the control of DOT. But Johnson said he will insist that the visitor center portion be the responsibility of Interior, because the National Park Service, a division of Interior, is the best qualified to run it.

Despite that reservation by the Carter administration, Mendelsohn hailed the compromise.

"We inherited a mess" from Republican administrations, said Mendelsohn, a former supervisor of the city of San Francisco. "We had a parking garage you couldn't park in, a train terminal you could barely walk to, a visitor center that no one visited, and a beautiful building that was falling apart."

"Once completed," he went on, "instead of a white elephant, the city will have one of the best intermodal terminals in the country." CAPTION: Picture 1, The parking garage would be finished for 1400 cars under agreement reached this week. By John McDonnell - The Washington Post; Picture 2, At right is the concourse where the train station would be located. Once part of the Visitor Center, the area is boarded up to save heat and light. By Tom Allen - The Washington Post; Picture 3, The National Visitor Center was devised to help crowds of tourists for the Bicentennial. Few used it. By John McDonnell-The Washington Post; Mapland 2, At top is a diagram of Union Station at present. Below is proposed diagram. The Washington Post.