The MBTA board voted Monday to back substantial investment in the commuter rail, calling for electrification of the system and more frequent service through the most dense corridors in a step that transportation advocates praised as historic.

The Fiscal and Management Control Board adopted unanimously resolutions calling for the T to develop a "commuter rail transformation office" within three months that will then oversee long-term work to electrify most of the network and run trains with 15-minute headways between key stations in a regional or urban rail model.

The decision still leaves questions unanswered about how the MBTA will pay for the changes, which will carry a price tag in the billions. Onlookers and activists who have long called for railway improvements complimented the board for supporting a plan to improve transit equity, take cars off congested roadways and reduce greenhouse gas emissions.

"It's definitely a win," said Jarred Johnson, chief operating officer of the TransitMatters advocacy group. "It's a win for better service, it's a win for equity concerns, and I think they also heard the call loud and clear that commuter rail fares have to be much, much more affordable."

The improvements will start on three lines. One resolution the board adopted instructs T officials to prepare immediately to launch electrified pilot programs on the Providence/Stoughton Line, the Fairmount Line, and a section of the Newburyport/Rockport Line connecting Boston to Lynn, Revere, Chelsea and Everett, sometimes referred to as the "environmental justice line."

The resolution also calls for service on the Fairmount Line and the "environmental justice" section of the Newburyport Line to be as frequent — and at the same fare level — as rapid transit.

Although those pilots would be the first phase of a broader transformation, it is not clear when they might begin. MBTA General Manager Steve Poftak said Monday that it could take two to four years to acquire electric trains for the programs.

Transportation Secretary Stephanie Pollack raised concerns during the meeting about the board backing specific pilots as a starting point for the transformation without a clear sense of operating costs, fare revenue changes and other key factors.

But members stressed they believe there are "actionable" steps worth targeting as soon as possible, citing a sense of urgency around rider-centric improvements.

"Let's see what happens. Let's go at it," said Joseph Aiello, the board's chair. "If we keep studying and studying and studying and don't commit ourselves to going to implementation, we'll be here in 20 years again at the same point."

With Monday's vote, the board also urged the Legislature to act quickly on procurement reforms that Gov. Charlie Baker included in his multi-year transportation bond bill, which members said are crucial to ensuring success for the major infrastructure work.

Baker told reporters Monday that he believes "significant portions" of the commuter rail should be electrified "over the course of the next 15 or 20 years," but only if the grid can handle the capacity and if electricity would be cleaner than diesel.

"The grid, as it stands today, would be a close call," Baker said. "If we manage to get our hydro and our offshore wind and some of the other initiatives we're pursuing to improve the quality of our electricity and we have the electricity available to electrify the system, there are certainly parts of it that should be electrified, yeah."

Aiello had prepared four resolutions, but the board added a fifth at the suggestion of Vice Chair Monica Tibbits-Nutt creating a complementary bus transformation office that will oversee major changes to the bus network.

Tibbits-Nutt said the new approach is necessary because existing T efforts to upgrade buses have "failed."

Members made some alterations to the resolutions during the meeting. Brian Shortsleeve, a former MBTA General Manager and current board appointee, asked for language that explains how renovation work would dovetail with Keolis' contract to operate the commuter rail, which expires in 2022 but has two separate two-year extensions.

A T spokesman said the final text would not be available until Monday evening or Tuesday morning.

The board's vote came one week after a 22-member commission presented its findings on six different commuter rail investment plans, ranging from simply running trains more frequently to a "full transformation" that combined electrification, regional rail service and a North-South Rail Link.

Members opted not to embrace one of those alternatives, but several described themselves as supporting a "5.5" model somewhere between the fifth and sixth — the most substantial and expensive — options.

"The board embraced the notion that the commuter rail system needs to transform itself into a so-called regional rail, urban rail system providing more aggressive service for the region, trying to attract more riders and to significantly make a dent in available capacity in order to decongest the roadway systems," Aiello told reporters after the meeting, adding that members are "certainly in favor" of electrification.

Because the FMCB is seeking a combination of investments, the potential cost was not clear Monday. Alternatives 5 and 6, which members wanted to draw from, were projected to cost $10.6 billion and $28.9 billion, respectively, in the Rail Vision study.

Aiello's resolution said the three pilot programs would require about $1.5 billion, but the chairman declined to answer questions from reporters after Monday's meeting about the overall price tag.

The MBTA will spend about $8 billion on capital projects over the next five years, a record amount over that timespan, but likely less than the investment needed to implement the proposed changes. The T also still faces an operating deficit every year, even though the Baker administration has defended the funding levels and said the Fiscal and Management Control Board cut down the size of the annual shortfall.

House lawmakers are preparing to debate new revenue options "sourced from transportation for transportation" this fall, such as increasing the state's gas tax or the fees imposed on ride-hailing trips.

Some business groups are supporting the House's revenue efforts, and even the T's board has in the past called for new revenues to fund the system: Some members said in April, as they approved a budget that included unpopular fare increases, that the Legislature needs to identify additional funding sources for public transit.

Rail Passenger Association Northeast Field Coordinator Joe Aiello — who has no relation to the MBTA board chair — urged the board not to shy away from the price tag, describing it as the "baseline for what we need after decades of not expanding."

"If we wait five or 10 years from now to come up with a plan, it's going to be triple that," the advocate said. "This is the price we pay to have a world-class system if we want to call ourselves a world-class city."