With plaster chunks from the crumbling ceiling of Union Station on a table in front of them, exasperated members of a House subcommittee yesterday tried to find out why the roof of the national landmark train terminal is falling down.

"This is a classic disaster. What is the [Reagan] administration going to do?" Rep. Norman D. Dicks (D-Wash.) asked of the Department of the Interior, which operates the 73-year-old station through the National Park Service.

"There has been no thinking on this matter until the roof fell in," replied Dick Hite, a dupty assitant secretary at Interior.

Yesterday's inquiry was one of more than 40 congressional hearings on Union Station held over the past 13 years, first to plan the conversion of the station into a National Visitors Center and then to examine why the conversion had been bungled so badly.

Summing up the ignominious history of congressional and bureaucratic failures at the white granite station -- which culminated on Monday with the closure of the building as heavy rains poured through a leaky roof -- Hite hold the appropriations subcommittee on Interior:

"It's stupid when you look back at it. I'll admit that."

Rep. Sidney Yates (D-Ill.), chairman of the subcommittee, called top officials in Interior and the park service to yesterday's special hearing to "find out what is the state of the roof and where do we go from here." Yates, who was told it would take $10 million to fix the station's roof, appeared incredulous throughout the two-hour meeting.

"This is really a very ridiculous situation," Yates said, after reciting a brief chronology of government blunders in dealing with the once-great train station, which were revealed recently in The Washington Post. He mentioned that the government spent $9.1 million for an elaborate renovation of the imposing building in the mid-1970s, but only $69,000 for minimum temporary repairs to the roof. As a result of ignoring the roof, rain leakage has ruined nearly all of the $9.1 million in renovation work.

Last work Congress authorized $11 million to repair the roof and pay for other structural repairs, but the money has not been appropriated. Yates told the park service yesterday to come back to his subcomittee on March 6 with detailed information on how the $11 million in emergency repair funds should be spent.

Congress has already spent or committed itself to spend $117 million in a project that has actually helped to destroy the architecturally imposing station and build one of world's most expensive, uncompleted parking garages. Under a 25-year deal with the railroads that owned Union Station, the federal government is bound to pay $3.3 million a year, beginning in 1976, to lease the building, even if it falls down.

Yesterday, one of the two original owners of the station, the Penn Central Corp., offered to sell the station to the government at a reduced price. Harvey Shipman of Penn Central told the subcommittee that if the government brought the station outright during the next six years, it could save up to $27.8 million, The Chessie System, the other co-owner of the station, also was said to favor the offer.

In the Senate on Tuesday, a $40 million bill was introduced to renovate Union Station, converting it back to the working train terminal it was before Congress got involved. If passed, that measure could bring the total cost of the federal government's dealings at Union Station to almost $180 million.