CHICAGO — The Regional Transportation Authority of Illinois board on Wednesday tapped its chief financial officer, Joe Costello, to serve as interim executive director beginning next month until a replacement for retiring director Stephen Schlickman is found.

It is a time of leadership transition for the agency. The board last month elected John Gates Jr. to serve as its new chairman, filling a vacancy left by Jim Reilly’s resignation last spring to serve as special trustee at the Metropolitan Pier and Exposition Authority.

Schlickman announced early this year that he planned to leave the agency after his fifth anniversary in October to return to his private transportation policy and finance consulting business.

The board’s search is ongoing, while Costello will also continue in his fiscal ­duties.

“The RTA plans to work harder than ever to ensure we continue our transparency in providing the most effective service possible,” he said in a statement in which he alluded to the region’s challenging economy and increasing need for public transit.

Costello joined the RTA in 1995, steering its finances through both operational and capital struggles.

The agency — which provides oversight of the Chicago Transit Authority, Metra commuter rail, and Pace suburban bus service — received an infusion of new cash from a sales-tax increase last year, but sharp declines in tax collections and delays in state aid have strained its operations.

On the capital side, the RTA recently received $442 million in long-awaited Illinois bonding proceeds for capital projects, representing the first installment of $2.7 billion of transit funding in the state’s capital budget.

The RTA’s $2.3 billion of debt is rated AA-minus with a negative outlook by Fitch Ratings, AA and stable by Standard & Poor’s, and Aa3 and stable by Moody’s Investors Service.

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