U.S. Secretary of Transportation Drew Lewis told a congressional committee yesterday that his department wants to spend $70 million to convert Union Station back to a railroad terminal, fill it with shops and restaurants and finally complete the parking garage behind the station.

Lewis asked for no money yesterday, saying final cost estimates are still being worked out, but he outlined the new plan, which also calls for building two new high-rise office buildings across F Street from the 74-year-old station.

He said a $1.3 million consultant's report had confirmed "that the building is basically sound and that it is capable of being developed as a commercial property . . . The alternative to going ahead with development is to mothball the station."

The plan is the latest in a series of proposals developed by federal officials, who have spent or committed more than $117 million to convert the once grand station into a cavernous and empty National Visitors Center and to construct the still-unfinished parking garage nearby.

A small replacement train station was built behind the visitors center several years ago. Yesterday's proposal calls for demolishing that station and restoring the railroad tracks all the way to the main building.

The Union Station building at First Street and Massachusetts Avenue NE, has been closed for safety reasons for more than two years, since heavy rains pouring through its roof caused chunks of plaster to fall from the ceiling.

Roof repairs costing $8 million, which were to be completed by the end of the year, just got under way last week and will not be finished for 18 months to two years, federal officials said yesterday.

The plan envisions converting the first floor of the station into a variety of restaurants and shops similar to the highly successful Harborplace in Baltimore and Quincy Market in Boston. The upper floors would be converted to office space.

The station would be reopened in 1987 under the plan, with the garage opening two years earlier. Congress must approve the plan and funds before work on the proposal can proceed.

The DOT plan calls for development over a five-year period. The first stage would be the construction of one or two new office buildings, each seven to eight stories high, on two small parks located between First and Second streets, Massachusetts Avenue and F Street. Much of the park land has been paved and now serves as a parking lot for congressional staff members.

To reduce the federal cost of the station's renovations, the plan calls for the government to lease the park land to a private developer who would construct the office buildings. The new buildings would total about 500,000 square feet and contain 654 parking spaces.

Completing the garage behind the station, which has been delayed for years by cost overruns, mismanagement and bureaucratic battles, would provide parking for an additional 1,296 cars.