PROVIDENCE — Because of poor design and construction and lack of maintenance, the underground parking garage at the Providence railroad station has suffered so much structural damage caused by leaking water that the state Department of Transportation says it might have to be closed.

The 360-space garage is a key transportation facility whose importance is likely to increase. It’s the most convenient station parking and is full of commuters’ cars on weekdays. The station will see more use, and presumably need more parking, when the state extends rail service south of Providence over the next two years.

“This is a critical piece of infrastructure,” said Stephen M. Devine, the DOT’s chief of intermodal planning and the coordinator of its rail program.

Transportation Director Michael P. Lewis said he thinks repairs could cost $10 million. The DOT has asked the federal government for $400,000 to assess the garage’s problems and design repairs. The state’s chances of getting the money are uncertain.

“The rehabilitation of this garage is necessary to avoid its closure,” Lewis said in a June letter to Joseph H. Boardman, the president and chief executive officer of the National Railroad Passenger Corp., or Amtrak. Both Amtrak and the commuter rail service operated by the Massachusetts Bay Transportation Authority use the station.

Lewis’ letter and an application for federal stimulus program money for repairs refer to “poor design and construction” along with “poor maintenance” as causes of the structure’s problems.

Inside, there are water streaks on the walls, puddles on the floor and broken pipes in the drainage system. When it rains outside, one employee says, it rains inside, too, with water leaking through open joints between the garage’s concrete structural elements.

Lewis’ letter said the leakage has caused extensive damage to the garage’s upper deck “due to a combination of poor design and construction of the garage/station interface,” and also due to “poor maintenance at the street level.” An application for money under the federal stimulus program, meanwhile, says Amtrak has identified the station as needing improved and expanded parking facilities.

Closing the garage would eject the commuters who use it, about 70 percent of them using Amtrak and the other 30 percent taking MBTA trains to Boston.

Closing it, the state said in the application, would affect a significant number of rail users and affect traffic and parking on nearby streets. Devine, however, said he knows of no immediate reason to close the station.

If it does close, the disruption would occur as the state is substantially expanding rail service. The DOT is extending lines south from Providence to Warwick and Wickford, creating commuter service inside the state and beefing up service to Boston.

Devine said the DOT has no formal responsibility for the garage, but has taken the initiative because of its importance.

Gilbane Building Co. built the garage in the mid-1980s as part of the Capital Center Project. “Twenty five years is a long period of time. When we handed it over, it was fine,” a company spokesman said. But given the situation, he said, “‘No comment’ is the best thing for us to do.”

The garage’s ownership is complicated.

“It’s like a layer cake,” said Charles Meyers, who bought the garage in 2007. His company, MetroPark Ltd., has 14 parking lots and garages in and around downtown.

Working from the bottom up: The ground underneath the garage is owned by Capital Properties, which leases the land to Meyers’ Park Row Properties. Meyers’ company owns the garage, which consists of three concrete decks enclosing two levels of parking. The top deck serves as the roof of the garage.

At ground level on top of that, above the garage and adjacent to the railroad station, are city streets, Park Row and Park Row West, surrounding a pedestrian plaza whose concrete is crumbling. That’s apparently the city’s responsibility.

Meyers wants the city to make repairs above the garage. “The city is responsible for repairing it, and it never has been repaired,” he said. The city government agrees, up to a point.

“That is city property, and the city is responsible for maintaining it,” said Alix Ogden, the city’s director of operations. “We know the scope of the issues, and we’re looking for funding for it.”

However, she said, “The city doesn’t have taxpayer dollars to put forward to that.”

Devine said he doesn’t know what will happen if the federal money isn’t available, but that the DOT “would still play a role in pulling all the parties to the table.”

blandis@projo.com