Correction: An earlier headline on this article incorrectly stated the cost of the upgrade.

The Forest Glen Metro station is seen this past summer. Metro officials said in a new report that the transit agency needs to take the next steps in planning for future population and job growth in the Washington area. (Katherine Frey/THE WASHINGTON POST)

Metro’s top managers are proposing a new rail tunnel under the center of the District, a second tunnel under the Potomac, and they estimate the transit agency will need $26 billion over the next three decades to pay for those and other improvements to an aging system that is falling behind the region’s needs.

The proposed new rail tunnels — one under 10th Street to Thomas Circle and another between Rosslyn and Georgetown and on to Thomas Circle — would be massive undertakings. The projects would require major financial commitments from local and federal governments and would take several years to plan and several more years to complete.

It is not the first time Metro has talked about such new tunnels, but in making them a central piece of the transit agency’s new strategic plan, Metro’s top managers are seeking to focus public attention on what the agency says is a long-term threat to the region’s economic growth.

The 49-page strategic plan is to be presented Thursday morning to Metro’s board of directors, which has yet to endorse the blueprint.

“If we don’t do something about what’s coming at us in terms of the region’s growth, we will have such a crowded system that it will create its own set of problems with regard to safety and infrastructure,” Richard Sarles, Metro’s general manager, said in an interview. “Now is the time for the region — with Metro in the lead — to begin talking about projects that make the most sense for making long-term investments.”

Thinking long term for Metro

The continuing population growth in the District and its suburbs, especially among young people who choose not to own cars, has made Metro’s long-term capacity an existential issue for the transit system, one of the busiest in the nation.

Along with the proposed tunnels, Metro says it must spend billions of dollars to upgrade the rail system’s electrical grid to handle more eight-car trains and must add pedestrian corridors between transfer stations such as Metro Center and Gallery Place. Above ground, Metro needs to buy more buses, build more bus garages and create bus-only lanes on busy downtown streets.

Dubbed “Momentum,” and 18 months in the making, Metro’s new strategic plan catalogues the system’s needs and renews the long-standing argument for Metro to have a dedicated funding source, just as many big-city transit systems do.

Metro’s lack of capital investment in the past decade has been blamed on that lack of dedicated funding, and planners say that unless that changes, there is little hope of executing the ambitious strategic plan that will be formally unveiled Thursday.

A new Metro line is being built in Northern Virginia, but it is being constructed for Metro by the Metropolitan Washington Airports Authority, with revenue from the Dulles Toll Road financing a significant part of the line’s $5.6 billion cost.

No such obvious source of financing exists for the new rail line and tunnels proposed in Metro’s new strategic plan, and the plan does not specify how the agency would finance the rail expansion and other costly improvements.

In Maryland and Virginia, the state governments have struggled for years to come up with ways to ensure long-term funding for transportation.

Metro released a strategic plan that will guide the transit agency’s decisions over the next 10 years.

Unlike other transit agencies in New York, Boston and Los Angeles that depend on some level of dedicated funds from specific taxes, Metro receives contributions from the District, Maryland, Virginia and the federal government for its operating and capital budgets, which total $2.5 billion.

Shyam Kannan, Metro’s chief planner, said it will take a “reliable, sustained stream of capital funding from a combination of local and federal” moneys to pay for the slew of proposed projects.

Metro’s system, including Metrobus, ranks as the fourth-largest transit system and second-largest rail system by ridership in the country, according to the American Public Transportation Association.

In the past decade, Metro has seen its ridership grow, and despite recent drops in riders from weekend track work as it rebuilds the aging system, officials said they expect ridership to increase 40 percent to 1.05 million weekday rider trips on average by 2040 as the new Silver Line comes on line later this year and other transportation projects in the region are underway.

Metrorail, which carried its first passengers in 1976, is entering a new era.

It faces the costly challenge of upgrading an aging system that hasn’t been well maintained over the years. Escalators break down frequently. Switches, tracks and other equipment are being replaced to meet National Transportation Safety Board safety recommendations. New rail cars have been purchased to replace the rail cars from the 1970s that were involved in the deadliest crash on the system in 2009.

Sarles said the $1-billion-a-year capital-spending project he has been aggressively pushing in the two years he has been at the helm shows the system is “middle aged and getting a facelift.”

Transportation experts said it is key to have an efficient and reliable transit system.

“We’re seeing a sea change in how people move about,” said Michael Melaniphy, president and chief executive of the American Public Transportation Association. “We see cities competing against each other to attract new business, and those that have access to public transportation have an edge.”

In its strategic plan, Metro proposes to spend $6 billion on its rail and bus systems by 2025.

Planners said running all eight-car trains on its five rail lines would help trains carry at least 35,000 more passengers an hour during weekday rush hours. But to do that, Metro needs to spend $2 billion to increase its power system, add more space at rail yards to park trains, and buy more rail cars.

Busy stations such as Gallery Place, Metro Center, L’Enfant Plaza and Union Station need additional mezzanines, stairs, elevators and escalators. Metro plans to build new underground pedestrian tunnels between busy stops such as Metro Center and Gallery Place.

One of the biggest ideas in its plan is to try to relieve congestion through the Rosslyn tunnel where Metro is already at its maximum capacity of 26 trains per hour. Riders on the Blue Line have expressed frustration of getting fewer trains per hour to make way for the new Silver Line.

To help alleviate the problem, planners suggest creating what they call the “Rosslyn bypass.” New tracks would be built west of Rosslyn and near the Pentagon, allowing trains on the Orange Line to go south into Virginia on the Blue Line and east over the Yellow Line bridge.

New stations would also be added at Rosslyn and Pentagon.

Planners also want to spend $600 million for bus service improvements, including new buses, garages and bus-only lanes along busy streets in downtown, including H and I streets NW between 13th and 17th streets.

The transit agency calls for an additional $20 billion worth of projects to be done by 2040, including building two tunnels.

One north-south tunnel, costing $2.7 billion, would be built under 10th Street SW and NW to Thomas Circle and would allow Green and Yellow line trains to run in separate tunnels. That would take pressure off the tunnel that runs between L’Enfant Plaza and Gallery Place. Another $3.3 billion tunnel would run east-west through Rosslyn to Georgetown and along M Street NW to Thomas Circle.

Zachary M. Schrag, author of a book on how Metro got built, said the rail system is pushing close to the 1 million riders a day it was designed to handle.

“It’s pretty much maxed out on the original design,” he said. “It is up to the next generation to plant the olive tree.”