The Atlanta region is poised to spend $36 million of its federal transportation funds on diesel locomotives owned by private rail companies so they will pollute less.

The Atlanta Regional Commission on Wednesday approved the money. Final approval now rests with the Georgia Regional Transportation Authority, which is expected to vote on the matter Sep. 10.

State regulators said they had little choice but to request spending the money on the companies, because of the setup of federal laws and regulations. The law may punish the Atlanta region for poor air quality that some of the old locomotives help cause, but it exempts rail companies from state regulation.

So the state can’t force the companies to buy cleaner engines, and has to offer the incentive of money.

Sonny Deriso, chairman of the GRTA board, said at the board’s meeting last month that before state regulators explained the situation, “we couldn’t believe that we were going to be asked to do this.”

Deriso said Wednesday he didn’t know how the board would vote, but that regulators had made “a compelling case” for spending the money. “I think this is one of those very difficult issues,” he said.

“It is one of the few big things that we have left here in the Atlanta area to go after” to reduce air pollution, said Heather Abrams, chief of the air protection branch at the state’s Environmental Protection Division.

Jim Kelly, a program manager at EPD, said retro-fitting or replacing the engines now would be a big, immediate help to Atlanta’s air quality score, and paying for it seemed to be the only way.

Kelly said that unlike the state, the federal government could regulate rail companies. The federal Environmental Protection Agency has a new rule that requires new or overhauled locomotives to be cleaner, but Kelly said it would be about 30 years before all engines had been upgraded.

“It is frustrating” as a state planner, Kelly said. “I have to put together a plan” to improve air quality, he said, “and you have this part of the puzzle you can’t do anything about.”

A spokeswoman for the federal EPA, Cathy Milbourn, said the agency’s regulations “across the board” required changes only to new and overhauled engines. She said the new federal rule would lessen pollution from the companies’ locomotives, and “we’re not giving them a pass.”

Representatives of two railroad companies involved in the Georgia program, Norfolk Southern and CSX, each said they were making their own strides in decreasing pollution, and that the Atlanta locomotives were a way to partner with the government for the public good.

Asked why the government shouldn’t make the companies pay all the costs, Gary Sease, a spokesman for CSX, said, “The question might be posed better to government entities that are making these cost-sharing opportunities available.”

Federal funds that the region could otherwise use to retrofit school buses or help fund mass transit will pay for the upgrades, said Atlanta Regional Commission transportation planning chief Jane Hayse. It’s to be paid out over four years.

The rail companies must also pay part of the cost, probably 20 percent to 50 percent, Kelly said. The state still has to negotiate the companies’ percentage with them.

The new locomotives have a cleaner diesel technology and should reduce their direct emissions of a key pollutant by about 90 percent, said Michelle Bergin, an environmental engineer at EPD.

Atlanta officials hope to avoid another crisis like the one in the 1990s.

At that time, the Atlanta region couldn’t spend federal funds on road expansion for more than a year because it had poor air quality and insufficient plans to improve.

Tad Leithead, who chairs the Regional Commission committee that dealt with the rail funds, said that it was “frustrating” to have to put so much money toward private companies while the region was dealing with increasingly dire transportation funding shortfalls. But the benefits appeared to be clear, he said, and, “It’s the hand we’ve been dealt.”