A new plan to rescue Union Station was passed without opposition in the Senate yesterday. The bill would provide for completion of emergency repairs to the dilapidated beaux arts building and its possible conversion into a combination rail and commercial complex.

A congressional aide said House members have agreed to bypass committee consideration and take the measure directly to the floor. Technical objections that House members had raised to the original bill have been satisfied by Senate amendments, the aide said, and the outlook for passage in the House is good.

Congressional concern over the 74-year-old structure, now officially known as the National Visitor Center, became pronounced in February after flooding and falling plaster forced its closing as a safety measure.

The Senate's bill calls for completion of an $8 million roof repair job already under way, with the government authorized to make further repairs if necessary. It also releases $40 million to the District government to complete a parking garage on which work was halted in 1976.

All this work will be covered by previously appropriated funds. The only new money in the bill is up to $1.7 million to buy out the lease with railroad interests that own the station. Control of the building would pass from the Interior to the Transportation Department.

The plan provides for two detailed studies, one into the station's structural shortcomings, and a second into the feasibility of contracting with private developers to build stores, offices and other commercial ventures in and around the station. However, the plan specifies that trains must continue to use the station.

The Senate plan is the latest expression of congressional concern for Union Station, opened in 1907 and once ranked among the grandest rail gateways in the world. After World War II, however, it began to decline.

In 1968, Congress ordered that it be transformed into the Visitor Center, a first stop for tourists arriving in Washington. Some $46 million was spent on the project, but the station never caught on in its new role and continued to deteriorate structurally.

A Senate Environment and Public Works Committee report condemned the Visitor Center project, calling it "an embarrassment to the Congress and the Executive Branch alike," according to United Press International. The project, it said, "resulted in the near ruin of a national landmark."