The House Committee on Public Works and Transportation threw out the recommendations of the Carter administration and its own subcommittee yesterday and authorized $36 million to complete the long-delayed National Visitor Center at Union Station.

Amtrak and commuter railroad passengers would continue to be consigned to the "new" Union Station out back, where the decor resembles a motel lobby. The administration had wanted to reclaim about half of the original Union Station building for passenger service and leave the rest as a tourist and visitor information center.

The committee bill now goes to the full House. The bill clearly reflects the will of chairman Harold T. Johnson (D-Calif.), who was a member of the committee when it originally approved the visitor center concept for Union Station in 1968.

"This proposal," Johnson said, "would cost less (than the administration plan) and would be the greatest good for the greatest number of people."

Since the National Visitor Center concept was approved in 1968, about $46 million has been spent to erect the skeleton of a large, uncompleted parking garage; to construct a multiscreen slide show in a pit in what used to be the main hall of the old railroad station; to build a carpeted, elevated plywood exhibit area on what used to be the old station's con-course, and to construct the lobby out back.

Construction of the Union Station-Visitor Center complex was halted in 1976 when the money ran out, largely because of huge overruns on the parking garage. Since then, extensive structural damage has been found in east and west office wings of the building and along the front portico because of a leaky roof.

Under the proposal approved yesterday, the garage will be completed, but with 1,200 parking spaces instead of the 4,000 originally envisioned.

Rep. Norman Y. Mineta (D-Calif.), chairman of the Subcommittee on Public Buildings and Grounds, fought valianty for the compromise he had worked out with the administration. However, there was no way he could beat the Public Works Committee chairman in an election year on a project that has no effect on anybody's congressional district, because it is in the District of Columbia.

"This will lose about 34 to 4," Mineta joked with staff members before the committee session began. The final vote in favor of the Johnson-backed amendment was 35 to 5.

The administration compromise, worked out by Transportation Secretary Brock Adams and Interior Secretary Cecil D. Andrus, would have cost $52.8 million but would have required only $30 million in new money. The rest would have come from already appropriated funds earmarked for improving rail facilities in the northeast corridor, between Washington and Boston.

About $2 billion is being spent in that program. One result, Mineta said, will be that the newer, smaller Union Station will not be adequate to handle the crowds. He said that only 57,000 square feet are available in the lobby, but that 90,000 feet will be needed by 1990.

Furthermore, Mineta said, the bill approved yesterday authorizes construction that will cut off expansion. A three-lane roadway would be built between the visitor center exhibits and the railroad terminal. Tracks are to the north and east, the parking lost is to the west.

"The complex advocated by Cairman Johnson does nothing but perpetuate a mistake," Adams said yesterday. A spokesman for Andrus said, "We certainly are disappointed."

The Senate is expected to hold hearings on Union Station shortly.