FOLLOWING THROUGH ON A CAMPAIGN PROMISE, Gov. Charlie Baker on Tuesday sought environmental approval for a phased approach to South Coast Rail that will allow service between Boston’s South Station and Fall River and New Bedford by late 2022.

The original plan was to extend the Stoughton commuter rail line from its current endpoint at Stoughton through Taunton to Fall River and New Bedford. While that project’s “complexities continue to be addressed,” the Baker administration is proposing to launch service earlier by extending the Middleborough/Lakeville commuter rail line to Pilgrim Junction and then over to East Taunton before heading south along the originally proposed route to Fall River and New Bedford.

State officials estimate the cost of the Middleborough/Lakeville phase one would be $935 million, with 85 percent of the amount going for service from Taunton south to Fall River and New Bedford. Completing the original South Coast Rail plan, which calls for electrifying and extending the Stoughton commuter rail line, is estimated to cost an extra $2.3 billion for a total cost for both phases of $3.2 billion.

Officials project phase one, which will use diesel locomotives to provide 13 round trips a day, will generate 3,220 daily one-way trips between South Station and New Bedford and Fall River.

For comparison purposes, the Middleboro/Lakeville line currently generates 5,249 daily one-way trips, while the Fairmount Line, the commuter rail system’s least-used line, generates 1,900.

Officials said the ride between South Station and Fall River and New Bedford will take between 91 and 95 minutes one-way, or a little over three hours roundtrip. The officials said the roundtrip will save passengers about an hour compared to a roundtrip by car.