MARTA has been locked inside Fulton and DeKalb counties since the agency rolled its first buses 42 years ago.

The MARTA referendum under consideration in Clayton County, then, takes on significance far beyond the southern metro county. It could become a defining moment in the history of the transit system and the Atlanta region — the moment when the region began to embrace the Metropolitan Atlanta Rapid Transit Authority.

A voter-approved penny sales tax in Clayton would expand MARTA beyond Fulton and DeKalb, add as much as $49 million a year to the transit agency’s bottom line, and bring bus service, heavy rail and possibly even commuter rail to Clayton. And, MARTA Board Chairman Robert L. Ashe III told the Clayton County commission Tuesday night, it would give his agency a chance to show other metro counties how they, too, could welcome MARTA.

“We intend to do everything in our level power to get it right and to be a shining example of why other jurisdictions might want to take a look at joining our system,” Ashe said.

Former Atlanta Mayor Sam Massell, now president of the Buckhead Coalition, has been a strong supporter of MARTA since he urged Atlantans to approve the system in 1971. He believes the tide of public sentiment began to turn in favor of MARTA after Keith Parker came on board as the new CEO and general manager in 2012.

“I don’t know what magic button he has, but it is a magic button he pushed when he got to Atlanta,” Massell said. “I see it everywhere I go. I know I feel it. He’s getting along with the Legislature, and now there’s another county stepping up to the plate.”

A bill passed by the General Assembly this year enables Clayton residents to vote on whether to pay a sales tax on top of the existing 7 percent sales tax to start a transit system. Initially there was some confusion about whether the only option was joining MARTA, or whether the county could explore other possibilities.

Commission Chairman Jeff Turner said Tuesday that the county now believes the legislation only allows the sales tax to be used for joining MARTA.

But that doesn’t mean the referendum is a sure thing.

Clayton could pursue other options, such as a county-funded system or a public-private partnership, Turner said. The county could also consider just approving a half-penny sales tax, but that would mean that MARTA would provide only bus service.

Clayton County Commissioners have until July 7 to decide whether to hold the referendum in the Nov. 4 general election. Otherwise, the opportunity afforded by the new law will expire.

In a presentation to the Board of Commissioners on Tuesday, Parker outlined the mass transit agency’s recent successes while making a case for why the county should join MARTA. Those successes included returning to fiscal solvency last year, instituting a new customer code of conduct, enhancing security and upgrading stations and equipment.

Parker said Clayton would be instrumental in shaping its own routes and service plans. And, the county would be tapping into MARTA’s existing $6 billion infrastructure network in DeKalb and Fulton, Parker added.

Rex resident Larry O’Keeffe said transit is badly needed in Clayton. Several of his employees at an airline catering company had to quit their jobs at Hartsfield-Jackson International Airport after C-Tran closed. But O’Keeffe said he’s not convinced MARTA is the answer.

He’s concerned that some of the money that county would pour into MARTA could be diverted to DeKalb or Fulton.

He also worried that the county would have only two of the 12 representatives on the MARTA Board.

“I’m not anti-MARTA,” O’Keeffe said. “But before we take the leap, we need to have concrete in writing what our rights and privileges are.”

College Park resident John Duke closed his veterinary practice after C-Tran service ended because he could no longer ride the bus to work. Duke favors joining MARTA. He thinks the agency would act quickly to bring rail service through Hapeville to Clayton.

“Fulton and DeKalb resources will help pay for a (rail extension), so you’re getting a pretty good deal,” Duke said.

As part of an ongoing transit feasibility study, the county has held a series of public meetings to ask citizens what they want.

Commissioners will hear the results of those sessions at an as-yet unscheduled meeting in June, Turner said.

Turner, who campaigned on restoring transit service to Clayton, declined to say whether he wanted the county to join MARTA. Since Clayton’s bus service, C-Tran, halted in 2010 because of budget cuts, Clayton is the only core metro Atlanta county that lacks its own transit service.

“I have confidence in Parker’s ability to run MARTA,” Turner said. “What I’m looking for is whether or not MARTA is a good fit.”