Neither Costello nor RTA Chairman John Gates Jr., who appointed him in 2010, responded to requests for comment.
The RTA’s 16-member board has repeatedly split along city-suburban lines over allocating shares of mass transit funding to the CTA, Metra and Pace, which the RTA oversees.
In September, for the second year in a row, the RTA missed a state-mandated deadline to issue 2014 budget goals for the CTA, Metra and Pace when Chicago Mayor Rahm Emanuel’s appointees blocked passage.
The RTA leadership also has been at odds with the CTA on oversight issues. Gates directed the RTA’s auditor to review the trouble-plagued Ventra rollout, criticizing the problems as a “systemic failure.”
Before he was appointed executive director, Costello served as chief financial officer since 1995. Costello replaced Steve Schlickman.
The announcement of his departure came just two hours after the RTA canceled a special meeting scheduled for Wednesday of its Compensation and Human Resources Committee. The reason for the meeting, announced Monday, was unclear but members were scheduled to have a closed session to discuss “personnel.”
Several members of the committee did not respond to calls for comment.
RTA board member Donald Totten, who is not on the panel, said Tuesday that Costello’s resignation came as a surprise to him. Totten could offer no explanation why Costello would step down, or if he was being forced out.
“This came out of left field,” Totten said. “I just think this is totally (a decision) on his part. For whatever reason, he’s had enough.”
Totten praised Costello’s financial acumen, saying his departure would leave “a big hole” at the agency. “You can’t replace that kind of knowledge overnight,” he said.
A longtime transit critic, state Rep. Al Riley, D-Olympia Fields, said Costello likely made a “personal decision” but added that he had no insight into his departure.
Riley, who on Tuesday announced his own appointment as chairman of the House Mass Transit Committee, said one of his goals would be to ensure that the transit agencies provide an equitable level of service through the metropolitan area.
Riley pointed out how the CTA provides direct train service to some north and west suburbs, including Evanston, Skokie and Oak Park, but not to the south Suburbs.
“The Red Line stops at 95th Street,” Riley said. “How long do you see that and play stupid?”
State law calls for the RTA chairman to appoint the executive director with the concurrence of 11 of the board’s directors. The law also requires that the executive director “be an individual of proven transportation and management skills.”