Facebook has entered into negotiations with the San Mateo County Transit District to improve the Dumbarton corridor, a critical connection point between the company’s bayfront headquarters in Menlo Park and cities on the east edge of the bay such as Fremont and Newark.

The project could include renovating a defunct rail bridge over the bay, built in 1910, that parallels the existing, congested bridge for cars and trucks.

The negotiating agreement, approved Wednesday by the transit district’s board, will allow Facebook and a public infrastructure investor, Plenary Group USA, to begin working on a plan to improve the corridor.

The transit district, known as SamTrans, said rail line improvements from the Peninsula to Newark — including fixing the old bridge — would cost slightly less than $1 billion. Funding could come from public and private investment, according to a SamTrans report, though it did not specify the breakdown.

A rail line across the water would help fast-growing Facebook recruit employees who live in Newark, Fremont and other East Bay communities but may be intimidated by the current jam on the Dumbarton road bridge. The rebuilt line could connect into the existing rail network.

The project could also include bicycle and pedestrian facilities as well as “enhanced bus service,” the report said.

The Dumbarton Rail Bridge stopped operating in the 1980s, and SamTrans bought it and some nearby areas in 1994. But Facebook is the first group, public or private, that’s shown interest since then in helping fund a reintroduction of rail, the report said. The company contributed $1 million two years ago to a study on area traffic improvements in partnership with SamTrans.

Members of the SamTrans board said the public-private partnership could set an example for how to address congestion across the region.

“I’m just really thrilled and excited to do this here in San Mateo County and take the lead on something like this,” Rose Guilbault, a SamTrans board member, said at Wednesday’s board meeting.

John Tenanes, Facebook’s vice president of global facilities and real estate, said in a written statement that the Dumbarton corridor was “one of many projects we are looking at to ease congestion in the Bay Area.”

The company wants to reopen the Dumbarton rail bridge, he said, but “it’s still early days and we need to understand the environmental impact of the proposal.”

Traffic has gotten worse across the Peninsula recently as priced-out employees commute long distances to get to work. Some cities are considering a new tax that would raise substantial sums from large companies like Google and Apple to fund transportation improvements.

Facebook wants to build a new campus called Willow Village in Menlo Park, which would eventually bring its staff to 35,000 people — roughly the city’s current population.

Ray Mueller, Menlo Park’s mayor pro tem, welcomed Facebook’s interest in reviving the Dumbarton corridor.

“It’s exactly the type of progress we need here in Menlo Park in order to make the type of development they are contemplating work,” he said.

Even as it hopes to move forward with the mammoth Menlo Park project, Facebook is also expanding rapidly across the Bay Area. It recently snapped up more than 1 million square feet of space in Sunnyvale, opened an office for its Instagram subsidiary at 181 Fremont in San Francisco, and leased all the office space in a 43-story office tower in San Francisco.

Wendy Lee is a San Francisco Chronicle staff writer. Email: wlee@sfchronicle.com Twitter: @thewendylee