Virginia will build a new rail bridge over the Potomac River connecting Arlington and the District to significantly expand commuter and passenger train service over the next decade, Gov. Ralph Northam (D) announced Thursday.

The state will own the new two-track span, to be constructed alongside the aged and overburdened Long Bridge. It will allow a 75 percent increase in frequency of Virginia Rail Express commuter trains and a doubling of Amtrak service between the District and Richmond, officials said.

The bridge will be built by as early as 2027 as part of a $3.7 billion investment that also includes adding new track in the Washington-to-Richmond corridor and acquisition of hundreds of miles of passenger right of way from the private company CSX.

The agreement with CSX is designed to make future expanded train service possible across the state, including from Doswell west to Clifton Forge, and from Richmond south to the North Carolina border, officials said.

The 10-year project will be paid for with local, state and federal money, Virginia officials said, but the state still needs a funding commitment from the District, Maryland and Northern Virginia.

This is the second time in just over a month that Virginia has committed to financing a major river crossing. Last month Northam and Maryland Gov. Larry Hogan (R) announced a joint, billion-dollar plan to rebuild and widen the American Legion Bridge. It was a major breakthrough for a long-desired expansion that promises to relieve congestion at the Washington region's worst traffic bottleneck.

In both deals, Virginia is making considerable investments in infrastructure that previously were the responsibility of others. CSX owns the Long Bridge, and most of it is located inside the District border, as the new bridge will be. But the new span is critical for Virginia to realize its ambitions to grow commuter and passenger train service as well as handle projected increases in freight transportation due to growth at the Port of Virginia.

“We have a once-in-a-generation opportunity to make our rail system work better for everyone, both in Virginia and along the entire East Coast,” Northam said. “This agreement will change the future of transportation in Virginia, improving our ability to move people and goods across the state, and opening up potential rail service in underserved parts of the Commonwealth.”

Calling it an “innovative agreement,” CSX President and Chief Executive Jim Foote said the partnership will allow the freight company to “advance our goals for increased safety, efficiency, and volume growth while meeting the public’s desire for more passenger rail service to relieve commuter traffic congestion in the I-95 corridor.”

Under the agreement with CSX, the state will purchase 225 miles of track and 350 miles of railroad right of way from the railroad for $525 million, including half of the right of way between Washington and Richmond. This will give the state more control over service increases.

The additional capacity connecting the District's Southwest Waterfront to Crystal City will also put Virginia on a path toward separating passenger and freight trains, which officials say will improve service reliability. It will allow VRE to introduce weekend service by 2026.

The Long Bridge, a 115-year-old double-track span over the Potomac, is at 98 percent capacity during peak traffic times. The bridge's two-track configuration also creates a bottleneck in the rail system as freight and passenger trains funnel from three tracks on each end of the bridge to two, slowing rail travel throughout the corridor.

“Until we expand the bridge we are not able to expand passenger service,” Jennifer Mitchell, director of the Virginia Department of Rail and Public Transportation said at the governor's transportation conference last month.

CSX will retain ownership of the Long Bridge and two tracks south of it. The state will own a third and fourth track from the new bridge that will carry passenger trains. Portions of the Interstate 95 rail line already have a third track, which Virginia will own under the agreement, and the state is pledging to build a third and fourth track in other stretches.

Rail advocates welcomed the news Thursday, calling the deal a “game changer” for Virginia.

It “puts Virginia in a position to control our own transportation destiny for decades to come,” Danny Plaugher, executive director of Virginians for High Speed Rail, a nonprofit that advocates for fast, frequent and reliable rail service in the state, said in an email.

“This really is nothing short of a game changer!” Plaugher said. “Our mobility will no longer be confined to the automobile but going forward Virginians across the Commonwealth will have multiple transportation choices to get from point A to point B including a substantial increase in intercity and commuter rail service.”

The bridge project, which will cost $1.9 billion of the $3.7 billion total, is undergoing a federal environmental review led by the Federal Railroad Administration and the District. A draft environmental impact statement released in September lays out a preferred five-year construction process that would keep the Long Bridge and build the second two-track bridge next to it to create a four-track crossing. A final recommendation is expected next year and construction could potentially begin within two years.

Virginia officials said they anticipate the bridge to be built and completed as early as 2027. The bridge along with $1.8 billion in rail investments south of it will be paid for with available resources that exist today, officials said.

Virginia will assume about one-third of the cost, using existing rail funding and additional discretionary funds available through the Commonwealth Transportation Board. Federal funds will cover about another third of the price tag, including the $45 million rail share from the Atlantic Gateway grant. Amtrak intends to invest $944 million, state officials said.

Virginia anticipates it will raise the final share from regional partners, including the District and Maryland, as well as VRE and other Northern Virginia transportation boards. The District and Maryland have pledged support for a bridge expansion, though it is unclear how much each would be willing to contribute.

The bridge expansion is viewed as necessary to allow the region to continue to attract big companies and to keep up with projected growth. Demand for transit between the two sides of the river is likely to grow with Amazon’s second headquarters in Crystal City and the addition of 25,000 jobs.

Until recently, Virginia rail officials had said they had secured $214 million from state rail funds for the Long Bridge project over the next six years. The Commonwealth Transportation Board, which regulates and funds transportation in the state, is considering allowing the state to sell bonds backed by toll revenue from the 66 Express Lanes to finance the Long Bridge.

District officials have said they see the project as critical to reducing traffic congestion into downtown. The expanded capacity also would open the possibility for Maryland MARC trains to travel past Union Station into Virginia and VRE trains to serve stations in Maryland. A federal review of the project estimates that eight MARC trains could cross to Virginia by 2040.

South of the Long Bridge, Virginia said it plans to invest in rail projects that would be needed to leverage the investment over the Potomac. Among those is a $185-million project to add a fourth track approaching the bridge from Alexandria, a project that is already fully funded in the Department of Rail and Public Transportation's six year capital plan.

Further south, plans are to add a third track from Franconia to Occoquan at a cost of $220 million and a $330 million rail bypass at Franconia-Springfield, which will allow the movement of trains when other trains are serving the station.

“Our vision is definitely on growth,” Mitchell said in an interview earlier this week. “We would like to get to a place where we have nearly hourly service between Richmond and D.C. We would like to continue expanding trains to places like Newport News, Norfolk and Petersburg. We would like to be able to continue expanding VRE service, because they do relieve congestion on the 66 and 95 corridors.”

The acquisition from CSX also includes the 186 miles of tracks in the Buckingham Branch Line between Doswell and Clifton Forge, which would allow Virginia to launch an east-west train route from Norfolk to the Roanoke area. As part of the deal, Virginia will also acquire rights to use the abandoned S-Line from Petersburg to Ridgeway, N.C., an investment that would facilitate plans for high-speed train system in the Southeast.

Under the agreement with CSX, the railroad will give Virginia permanent rights to increase the number of Amtrak and VRE trains operating on CSX tracks.

Today Amtrak runs five Northeast Regional trains to Richmond. Officials anticipate adding six additional daily roundtrips to Richmond and one extension to the Hampton Roads area.

Virginia Railway Express, which runs eight trains on the Fredericksburg line, will add five new roundtrip trains during the rush hour and introduce three roundtrips on Saturday and Sunday. The agreement, officials said, will also allow VRE to add special Friday evening trains to give Virginians the option to use VRE after hours.

Some trains could start as soon as the deal with CSX is finalized next summer and additional trains by 2026.

Robert McCartney contributed to this article.