THE STATE’S TOP ENVIRONMENTAL OFFICIAL on Monday gave the green light to a project that would phase in rail service from Boston to New Bedford and Fall River, with the first phase launching in 2022.

Matthew Beaton, the state secretary of energy and environmental affairs, issued his decision shortly after 5 p.m., clearing the way for extending the Middleborough/Lakeville commuter rail line to Pilgrim Junction in Middleborough and then over to East Taunton before heading south to Fall River and New Bedford. Service would be provided by diesel locomotives.

The second phase calls for electrifying and extending the Stoughton commuter rail line from its current endpoint at Stoughton through Taunton to Fall River and New Bedford. Baker administration officials say 85 percent of the estimated $935 million cost of the first phase would go for track work connecting Taunton to Fall River and New Bedford. That work is also needed for the second phase, which is expected to be completed in 2028. Together, the two phases are expected to cost $3.3 billion.

Beaton characterized the ridership projections developed by the Baker administration – 1,600 daily riders in 2030 and 3,900 in 2040 — as somewhat uncertain. He noted the Baker administration did not estimate a cost per rider. He said the new rail service would reduce bus ridership between the South Coast and Boston from 2,200 to 1,400 by 2030.

Gov. Charlie Baker opposed South Coast rail when he ran unsuccessfully against Deval Patrick for governor, but he reversed course when he ran again in 2014. In his state of the state address this year, he announced that “after more than three decades of lip service, we’re going to make commuter rail from Fall River and New Bedford to Boston a reality.”