Union Station, Washington's 73-year-old train station, was declared unsafe and closed yesterday by the National Park Service as heavy rains poured through the roof of the white granite structure, causing plaster to fall from the ceiling and ruining more than $5,000 worth of books and posters in the National Bookstore.

The imposing station, a national landmark that has deteriorated under a leaky roof for more than a decade while Congress and the federal bureaucracy have been unable to decide what to do with it, will be closed until further notice pending "a safety-structural investigation," said Jack Fish, director of the national capital region of the park service.

Thousands of grumbling train passengers yesterday were forced to walk in the rain more than 400 feet around the old Union Station building to Amtrak's "replacement train station" in the rear. In Union Station, which was built to be the finest railroad station in the world and is now formally called the National Visitor Center, rainwater puddled up to two inches deep on carpeted floors and small chunks of plaster fell from the barrel-vaulted ceiling.

The rail-imposed closure yesterday is but the latest sad twist in the troubled 13-year history of federal involvement at Union Station. During that time, Congress has spent or committed itself to spend $117 million in the conversion of the once-great station into the visitor center. The project, which includes one of the world's most expensive uncompleted parking garages, has been plagued by cost overruns, mismanagement and bureaucratic fighting.

A $2 million effort to fix a small part of the station's leaky roof is now only 5 percent complete and will not be done until the end of June.Even when it is completed, more than two-thirds of the building will continue to leak every time rain falls, according to the park service.

There is no other federal money available for emergency repairs on the building and, according to sources in the Department of Transportation, the chance of coming up with any funds under the budget-slashing Reagan administration is slim.

"The administration has not totally dropped Union Station as a priority," said one top DOT official. "But the problems there are so complicated, so big and so dirty that it will be a while before we figure out what to do, if anything."

Last year Congress authorized an $11 million emergency repair bill that would fix the roof and pay for other structural repairs, but the money has not been appropriated and it is not listed on any proposed budget before Congress. A key House member, however, said yesterday that he will try to push through an emergency bill to fix the rapidly crumbling building.

"Something has got to be done. We just can't let the roof cave in," said Rep. Sidney Yates (D-Ill.), chairman of the House Appropriations subcommittee on Interior and related agencies. Yates said he will try to find money for the station with a supplemental appropriations bill and that he will hold hearings on the matter in the next six to eight weeks.

Under a 25-year deal with the railroads that owned Union Station, the federal government is bound to repay $3.5 million a year beginning in 1976 to lease the building, even if it falls down because of rain damage.

"It doesn't make sense to me to be paying on an empty shell for the next 20 years," Yates said. "We've got to get a program and get a handle on this thing."

In the station yesterday, clearly dispirited employes of the park service surveyed rain damage after the building was closed to the public at 1:30 p.m. "We don't know how long the plaster in the ceiling will hold," said Gary Pieroccioni, chief of visitor services.

In the National Bookstore in the east wing of the station, hundreds of water-sogged, full-color posters of the scenic United States were drying in the one dry corner, and the sound of falling water echoed through the building.