MARTA plans to ask Georgia lawmakers next year to convert a portion of a proposed transportation sales tax for use to add rail lines, the transit agency’s chairman says.

MARTA Chairman Robert L. Ashe said the agency wants to use half of a proposed transportation sales tax to pay for transit expansion and to ask DeKalb and Fulton voters to extend the tax for 42 years, through the life of the current MARTA sales tax.

“That’s the only way we’re going to get [the expansions] built,” Ashe told members of the Rotary Club of Sandy Springs during their Oct. 26 meeting.

State lawmakers last year agreed to allow county voters to decide in 2016 whether to impose a penny sales tax for five years to pay for transportation improvements, Ashe said. MARTA wants lawmakers to allow Fulton and DeKalb voters to dedicate half of that penny to transit rail expansions and to extend the tax.

If the voters approve the change, he said, the tax could raise $4 billion that could be used to help finance construction of rail extensions north along Ga. 400 to Alpharetta and east along I-20 in DeKalb, and to connect the east and north train lines with a light rail through the Emory University area.

Without the sales tax, “we have no way to pay for” the rail line extensions, he said. Together, the projects are expected to cost $6 billion to $7 billion, he said. By dedicating sales tax proceeds to MARTA expansion and extending the tax, he said, the funds could be use to attract federal monies to finance the remainder of the cost.

Ashe said he was “cautiously optimistic” state lawmakers would go along with MARTA’s request.

He said the state transit agency had changed its reputation by putting its finances in order. The agency posted a $35 million surplus in its 2015 fiscal year, with about $443 million in revenues and about $408 in expenses, he said. As recently as fiscal year 2011, the agency had run $35 million in the red, he said. “Over four years, we have held expenses constant and at the same time, we have improved revenues,” he said.