A Metro train stops at the L’Enfant Plaza station. (Evelyn Hockstein for The Washington Post)

Metro said Wednesday that it will spend about $120 million to bring cellphone service and better radio communications to its subway tunnels, revamping a failed, years-old plan that originally called for four cellular carriers to bear the bulk of the cost.

The new deal between Metro and the four companies — under which the transit agency will install radio and cellular cables in its tunnels, paying for the project using capital-improvement funds — appears to be radically different from an earlier version, reached about eight years ago and discussed as recently as September.

The original plan called for a consortium of Verizon, T-Mobile, Sprint and AT&T to oversee and pay for installation of the cellular cable. At the time, the carriers seemed more than willing to make the large investment because they stood to profit from cellphone use by commuters in the second-busiest subway in the nation.

Now, long after the companies were unable to complete the work and the project fell dormant, Metro General Manager Paul J. Wiedefeld has announced its rebirth — but with the transit agency, not carriers, footing most of the bill.

The reason for the dramatic change was not immediately clear. A spokeswoman for the carriers declined to comment on specifics of the deal, and Metro said it would not release details without a written request under its public records policy, which requires processing time.

Jack Evans, chairman of the Washington Metropolitan Area Transit Authority’s board of directors, and board member Corbett A. Price, who have been especially vocal as financial watchdogs of the agency, said they had not been briefed on the project. They said they plan to question Wiedefeld about it at Thursday’s meeting.

The new agreement comes at a time of deep money problems in the transit agency. Stagnant fare revenue and the reluctance of Washington-area jurisdictions to boost their financial subsidies have put a squeeze on Metro’s operating budget. And the agency’s past mismanagement of federal grants has prompted federal officials to restrict Metro’s access to such funds, which are used to pay for such capital improvements as the cellular project.

Wiedefeld and new board members, including Evans and Price, have said that fixing those problems is a top priority.

Price, a businessman, said he didn’t “really have enough information” to explain why the financial burden has shifted. “But I do think the carriers have undergone a significant change in their business model, because the business hasn’t been as profitable as it used to be,” he added. “They’re having financial issues.”

In the years since the original agreement, around 2008, the cellular industry’s potential for growth has been intensely questioned during an era when the nation is saturated with smartphones. Like other carriers, the four in the consortium have been under increasing pressure to maintain high profitability, a situation complicated by the entry, or potential entry, of new competitors in the field.

As recently as September, when the new arrangement was being negotiated, Metro said a deal was close to being finalized under which Metro would handle the installation of cables in its 100-plus miles of tunnels. The carriers, they said, would pay Metro tens of millions of dollars for the work. If that was where the talks stood at the time, then the discussions took a drastic turn.

On Wednesday, Metro spokesman Dan Stessel would say only that “the carriers are contributing to the project with funding and materials. That offsets the $120 million to some degree.” But he declined to say how much. The consortium’s spokeswoman, Kate Jay of Verizon, said in an email: “We do not release capital or contract details.”

In announcing the revived project, Metro did not offer a completion timetable. Wiedefeld said it “will result in vastly improved radio coverage for Metro and the region’s first responders, as well as wireless-technology coverage within Metro’s tunnels,” allowing riders to chat on their phones, send and receive emails and texts, and surf the Web in what long has been a cellular dark zone.

“Engineering and production tests are already underway in a 6,000-foot tunnel segment between Glenmont and Forest Glen in order to develop project design specifications,” Metro said in a statement. An outside expert has been hired “to review Metro’s design and implementation plan . . . to accelerate the project schedule.”

The statement also said: “Metro must design and test the engineering of the project to set a schedule for an unprecedented installation of 200 miles of cable, which breaks down to 100 miles of tunnel multiplied by two coaxial cables.”

As the cables are being installed, cellular service will become available progressively in various parts of the subway rather than all at once, when the project is completed, Wiedefeld said. “We are looking for every opportunity . . . to deliver incremental benefits as we complete each segment and turn it over to the telecom providers,” he said.

In September, when Metro officials talked about the carriers paying for the installation, they hoped that the work would be finished by 2020. But Wednesday’s announcement mentioned no end date. The project was supposed to have been completed four years ago under the original agreement that called for the consortium of carriers to install the tunnel wiring.

But obstacles got in the way, including unforeseen logistical hurdles that the carriers could not handle, Metro said. Also, the consortium’s prime contractor went bankrupt. And a fatalcrash forced Metro to give safety-related infrastructure work higher priority.

The carriers met their first deadline — making cell service available in the 20 busiest underground stations by 2009. The remaining stations were wired later. But installing cell cables in the “challenging environment” of a tunnel proved to be more difficult and time-consuming than anticipated, Metro said.

Then, on June 22, 2009, a Red Line train crash near the Fort Totten station killed nine people. The numerous safety-related problems in the subway that came to light after the disaster required Metro to undertake a years-long overhaul of infrastructure.

Given the limited number of hours available for tunnel work while trains are diverted or the subway is closed, there was not enough time or room for cable installers to stay on schedule while Metro crews labored in the tight spaces underground, the transit agency said.

By the next deadline, in 2012, when the cable installation was supposed to have been finished, only small portions of a few tunnels had been wired.

In 2013, after the consortium’s prime contractor, Powerwave Technologies, went bankrupt, months of legal and financial wrangling ensued between the four carriers and the failed company. By the fall of 2014, officials said, Metro knew that many more months were likely to pass before the carriers lined up a new prime contractor.

So transit officials and the consortium began negotiating a revised deal, with Metro saying in September that it would take care of the installation work.

Then came Wednesday’s surprising announcement: “At an estimated cost of $120 million,” it said, “the underground portion of the work will be funded through Metro’s capital-improvement program.”