The Reagan administration is prepared to spend at least another $48 million to repair and improve Washington's Union Station, an Interior Department official told a congressional hearing yesterday.

In making the expenditure, the administration would abandon the Interior Department's abortive program to convert the station primarily into a National Visitor Center -- which has cost $17 million up to now -- and would turn the entire monumental structure back into a transportation station.

The fate of the 73-year-old station has been in doubt since Feb. 23 when heavy rains poured down through its roof causing plaster to fall from the ceiling and leading officials of the National Park Service, part of the Interior Department, to declare the station unsafe and close it to the public. Since then, railroad passengers have been forced to walk around the building to reach the Amtrak station located in the track area behind it.

Even before its closure, however, the station's future was uncertain, since no agreement had been reached among the two federal departments involved in the station building's operation, Interior and Transportation, and Congress, which must appropriate whatever funds are needed for a construction program.

Richard R. Hite, principal deputy assistant secretary of the interior for policy, who was the department's chief manager of the visitor center project, testified yesterday that the National Park Service plans to reprogram $8.1 million currently earmarked for construction work at five national parks around the country and divert it to repair of the Union Station roof. The parks are in South Carolina, California, Mississippi and New Mexico and have been delayed by local controversies, Hite said.

If the congressional appropriations committee approve the change in programming, Hite told the House Interior appropriations subsommittee that the money would be added to $2.4 million provided earlier by Congress and now being spent for roof reparis, and the station could be reopened for train passengers by Sept. 1.

Beyond that, Hite said, the Department of Transportation is drafting legislation for approval by the Office of Management and Budget and presentation to Congress that would give the Transportation Department full jurisdiction over Union Station. It would authorize spending another $40 million to complete the parking decks that remain starkly unfinished over the station tracks and to do other unspecified restoration work, possibly starting next year.

It remains an open question, Hite said, whether Union Station would be exclusively a railroad station or whether the old plan to include an intercity bus terminal and possibly heliport facilities might be revived.

He also raised the possibility that local Washington investors might install retail shopping facilities in the building.

Rep. Norman D. Hicks (D-Wash.), who presided at yesterday's hearing, asked Hite whether his testimony, still resented Reagan administration policy. "I think I can say that," Hite replied. "The facility needs to be completed. It is a national disgrace."