According to published reports, overtime costs at the CTA increased by $1 million last year.
The Chicago Tribune analyzed the agency's 2010 payroll records and found discrepancies in how the CTA calculates overtime and that some employees actively sought overtime to boost salaries.
According to the Tribune, CTA is the only public agency in Illinois that takes overtime into account when calculating pensions.
The Tribune says bus drivers received the biggest portion of overtime -- about 95 percent of approximately 4,600 drivers. One driver more than doubled his pay of $59,580.
A CTA spokeswoman says overtime is budgeted for.
(The Associated Press contributed to this report.)