It may be years, if not a decade, before construction begins on a proposed multibillion-dollar redevelopment of Union Station. But debate has already been heating up over a parking garage planned as part of the overhaul.

The proposed $8 billion expansion would add concourses and tracks, broaden the retail options and improve bus and parking facilities at Washington’s busiest transit hub. A replacement parking garage with nearly 1,600 spaces — about one-third less than what’s currently available — would be attached to the station, according to the plan.

Some D.C. elected officials and residents aren’t happy with the parking space and are urging planners and the project’s proponents to go even further in limiting parking. Some are proposing eliminating the parking option altogether.

“The amount of parking proposed runs directly counter to our efforts to reduce automobile travel,” said Drew Courtney, a member of the Advisory Neighborhood Commission who represents neighborhoods adjacent to Union Station. By keeping hundreds of parking spots at the terminal, he said, Union Station will send a message to commuters that it’s a destination to be driven to and from.

“More cars means more traffic, more congestion and more pollution,” Courtney said.

The Federal Railroad Administration (FRA), which owns the station, is leading the federal review of the project expected to be out for comment this spring and finalized by the end of the year. This will bring the expansion concept first announced in 2012 closer to construction, though proponents say the project may not be completely realized until 2040.

The project, proposed by the Union Station Redevelopment Corporation and Amtrak, promises a complete overhaul of the nation’s second busiest rail station — after New York’s Penn Station — whose many facilities date back to the station’s 1907 opening. It would triple passenger capacity and transform the overcrowded station into a hub for high-speed rail.

Union Station gets about 40 million visitors each year and is served by 85 to 90 intercity Amtrak trains on any given day. It is also the Washington region’s busiest transit hub — connecting Amtrak, Metro, Virginia Railway Express, Maryland MARC commuter trains and intercity and local buses.

“Union Station is really at the epicenter of the District’s plan for the 21st century,” former mayor Anthony A. Williams said recently, calling the redevelopment plan an opportunity to reach for “something very special.”

That something special doesn’t have to be a magnet for drivers, D.C. Council member Charles Allen (D-Ward 6) said in a letter to the National Capital Planning Commission (NCPC), which held its first review of the proposal on Jan. 9. The project as proposed, Allen said, overbuilds parking.

“While I appreciate that the preferred alternative does contemplate fewer parking spaces than in the current garage, I believe parking must be even further reduced at this dense, urban transit hub,” he said.

Parking would be reduced to 1,575 spots from the existing 2,200 under the FRA’s preferred construction option. This parking inventory would be in line with what is available in stations in Philadelphia and Boston, according to an analysis by the NCPC. New York’s Penn Station, however, and other stations across the United States and the world do not have on-site parking.

Having parking, critics say, discourages commuters from using transit. According to the NCPC report, nearly 1,400 of the 2,200 parking spots available at the Union Station garage are used by monthly parkers, meaning the parking isn’t serving train riders or people frequenting the terminal’s retail stores.

Beverley Swaim-Staley, president and chief executive of the Union Station Redevelopment Corporation, said the parking number in the proposal is based on estimates from traffic studies and current parking usage. It makes assumptions of reduced parking needs by monthly parkers in the future. But she said the number is not set in stone and the group will continue to work with the city, community and developers to find the right parking approach as the project moves forward.

“We want to make sure that we have sufficient parking if we all agree that parking is necessary,” she said. “But we are still trying to figure out what the parking needs will be.”

David Valenstein, a senior adviser at the FRA and the project leader, said the agency is working closely with District agencies to develop a good traffic circulation plan that’s responsive to traffic concerns. He said parking is viewed as essential to having “equitable access” at the terminal.

The proposed parking structure, he said, will accommodate rental cars as well as short-term and long-term parking at the station complex, Valenstein told the NCPC on Jan. 9. In the future, he said, the parking might accommodate autonomous vehicles “that will need to be readily available at the station.”

The draft of an environmental-impact statement set to be released this spring will lay out the parking needs and overall project impacts. After a comment period, the FRA is expected to release a final recommendation. That would be a final step in the federal review process to move the project forward.

The realization of the plan, however, may take decades. Once the federal approval process is complete, there will be “an extensive” design phase that is likely to take several years before construction, Swaim-Staley said.

Financing is still lacking for the multibillion-dollar project. Proponents say they anticipate construction will be done in phases and require multiple sources of funding, including government funding commitments.

Meanwhile, planners are busy finalizing the project concept that includes an east-west train hall, an access zone for pedestrians and skylights between H Street NE and the train hall, pedestrian and bicycle access improvements and improved pickup and drop-off areas for vehicles, new concourses and platforms and extended tracks. And the 1,575-spot parking garage above a newly built bus facility.

Amtrak, one of the project’s proponents, said while it supports all improvements to enhance the customer experience, parking is not essential to its operations at Union Station.

“We encourage our customers to travel to the station through modes other than private vehicle to park,” spokeswoman Kimberly Woods said.

In the coming months and years, more discussions are likely about traffic impacts and the station’s future parking needs, including whether any parking should be above or below ground, the latter a preferred option but one that could be challenging and costly to build.

“We should build the community that we want and what we want is one that encourages other modes of transportation, gives people other options and doesn’t encourage more travel by car,” Courtney said.