Creaky old Union Station, crippled by 13 years of government ineptitude, issued another death rattle yesterday when a corroded pipeline burst, dumping more than 10,000 gallons of water on the only tenants remaining in the station's rotting west wing.

The tenants, criminal investigators for the U.S. Park Police, said the early morning flood soaked more than $10,000 worth of video equipment, along with "certain pieces of evidence for active cases," darkroom equipment, fingerprint powder, books and manuals. Wheelbarrow-sized chunks of plaster fell from the second-floor ceiling, and water up to two inches deep collected throughout the park police offices.

"It doesn't even have to rain to pour at Union Station," said George Berklacy, a spokesman for the National Park Service, which runs the crumbling national landmark.

It's been a bad week for the white granite structure that was built in 1907 by the Baltimore & Ohio and the Pennsylvania Railroad to be "the finest railroad station in world." On Monday, heavy rains poured through the building's leaky roof, forceing the park service to declare the terminal unsafe and close it for the first time since World War II. On Tuesday, volleyball-sized chunks of plaster crashed down from the rain-soaked ceiling to the station's filthy carpeted floors.On Wednesday, a congressional subcommittee met to find out why the station was crumbling and was told by a high-lvel bureaucrat that "there has been no thinking on this matter [by the Reagan administration] until the roof fell in."

And early yesterday at about 2:30 a.m., a $10 coupling recently installed to connect two 73-year-old sections of water pipe came loose, causing more than 10,000 gallons of city water to gush down from the third-floor level of the station's west wing. It took one hour and twenty minutes to shut off the water.

"We are talking about an old water system. It just didn't hold," said Georgia Ellard, director of the station that the federal government renamed the National Visitor Center in 1976.

The west wing of Union Station has undergone water intrusion after nearly every rainfall in recent years. The roof leaks badly, and although the park service is now spending $2 million to fix other sections of the building's roof, there is currently no money for the west wing.

When they come to work at Union Station each morning, park police have grown accustomed to cleaning fallen plaster off their desks and emptying wastebaskets full of rainwater. Ironically, running water from pipes in the west wing offices had been nearly nonexistent prior to yesterday's rupture. Toilets in the offices had to be filled with water from a bucket to be flushed.

In the park police offices yesterday, which smelled of rotting wood and soggy woolen carpets, investigators were drying out video and photographic equipment in hopes that the water hadn't ruined it.

"We won't know what to throw out until it dries and we can plug it in," said Curtis Shane, an identification technician. As he spoke, water continued to drip from lighting fixtures and electrical outlets.

Shane said most of the fingerprint records and criminal files stored in Union Station were saved from the water. But investigators have yet to figure out if the deluge damaged evidence that will be needed in criminal prosecutions. The 27-member police unit, which moved into Union Station in March of 1980, investigates crimes that occur on park service property.

According to the park service, other offices for the police are being sought.

Money to fix the entire roof of Union Station, a job estimated to cost $10 million, has been authorized by Congress, but not appropriated. Rep. Sidney Yates (D-Ill.), chairman of the appropriations subcommittee that called the special hearing on Wednesday, has asked the Reagan administration to come up with a detailed plan by March 6 for spending the money.

Congress has already spent or committed itself to spend $117 million in the conversion of the station to a visitors center, a project that has been marked by mismanagement, cost overruns and outright deception.