Triumphant MARTA Board members broke into spontaneous applause Thursday after voting to add Clayton County to the transit agency’s service contract, one of the last steps needed to usher in the first expansion of MARTA beyond Fulton and DeKalb counties since its inception.

Two of the three participating MARTA jurisdictions (Atlanta, Fulton and DeKalb) also have to approve the contract before the deal becomes official, which is expected to happen by the end of this year.

Clayton residents voted by a landslide 74 percent margin on Tuesday to join MARTA by paying a 1 percent sales tax. The tax increase (from 7 percent to 8) will take effect in March and is expected to generate about $45 million a year for the next 33 years. As such, it will be the largest new funding commitment for public transportation in metro Atlanta in more than three decades.

MARTA General Manager and CEO Keith Parker said he was gratified by the high turnout in Clayton and the huge margin of voters who favored joining the transit system. He said his goal since taking over the top job in December 2012 has been to eliminate all the legitimate reasons why people would not want to invest in MARTA.

In the two years since, MARTA has cracked down on security with a strongly publicized “Ride with Respect” campaign to cut down on nuisance behavior, installed new surveillance cameras on trains and buses, expanded bus and rail service, and implemented extensive cost-cutting measures that put the agency back in the black. Parker said those moves have opened the door for other jurisdictions, such as Clayton, to consider joining MARTA.

“Three-to-one is a solid mandate and it shows people really want to join our team,” Parker said. “I think it’s going to be a historic partnership.”

Clayton County Commission Chairman Jeffrey E. Turner said MARTA is the key to unlocking economic growth and improving the quality of life for the community.

“Public transit is the critical link we need to bring quality job opportunities to the citizens of Clayton County,” Turner said in a statement Wednesday. “This is the catalyst to bring significant development to the south side of the region over the next few years.”

MARTA will offer limited service in Clayton starting in March. Full bus service will be phased in by the following year.

Half the money raised by the tax will be set aside for a future commuter rail or a comparable form of high-capacity service (such as bus rapid transit). Parker said that negotiations to use Norfolk Southern’s tracks for commuter rail are getting underway, but he declined to elaborate.

“The plan is not to do a lot in the media or in the public and just try to have some conversations with them,” Parker said.

Local transit advocates are hopeful that the state’s first commuter rail could be built from the Fulton city of Eastpoint to the Clayton city of Lovejoy with the money generated by the MARTA tax. However, Norfolk Southern has publicly expressed reservations about sharing freight tracks along one of its busiest routes with passenger trains.