Green Line maintenance workers pulled in on average more than $32,500 in overtime for most of 2015 — more than double that of those on the Blue Line and far outpacing all others doing maintenance on the lines, according to a Herald review of departments driving the beleaguered transit agency’s OT bonanza.

The analysis of T data — which covers most of last year, through Dec. 15 — comes as the agency has launched its own audit of its overtime spending over the past two years; in 2015 alone, it grappled with a ?$75 million tab for extra time for its 6,500 workers.

The Green Line included some of the most noticeable outliers, with the 121 employees working out of Riverside and the 40 from Reservoir averaging $34,134 and $27,564, respectively, in overtime, or more than $32,500 when taken together. Riverside alone accounted for $4.1 million in total OT costs.

By comparison, the 41 Blue Line maintenance workers averaged $15,410, and the Red Line’s Cabot shop, which also employs 121 workers, averaged $18,025 per employee. The 11 employees at the Mattapan shop, which also services the Red Line, averaged $21,276, while Orange Line workers clocked in on average $20,706 OT apiece.

T officials declined to directly address the discrepancy in pay on the Green Line, only to say it will be covered in their review.

But the huge OT tab caught the eye of one watchdog, who says it’s all about mismanagement.

“The overtime to me is unnecessary if they were managing the system well, which they’re not doing,” said Greg Sullivan, a former state inspector general now with the Pioneer Institute. “How long does it take to replace a part? How long does it take to do a particular task? The T has the ability to determine that under their contract … In my view the No. 1 target for reform in the T is in the maintenance program.”

They weren’t alone, however, in eye-popping totals:

• The T’s nine-man Signal Communications Maintenance department, which is responsible for the system’s thousands of phone lines and fiber optic cables, averaged a T-wide high of $42,199 in overtime per workers, nearly four times that of the average T employee, who took home $11,644. Electrical maintenance workers covering the T’s power system ($39,954) and those responsible for the system’s overhead wires and third rail power ($35,849) ranked just behind them.

• Transit police led all departments in total OT, with its 273 personnel racking up $7.9 million and 162,813 hours.

• Blue and Orange line operators — at $10,200 and $10,900, respectively — led bus and rail drivers, in some cases making nearly double the OT per employee than those in transportation on the Green Line and working out of various bus garages.

T spokesman Joe Pesaturo said the train operators’ OT was related to the agency’s $90 million winter resiliency plan, which included drivers often being used to help staff stations and provide customer assistance for bus shuttles when train service was suspended on nights and weekends.

The T spent $53 million of its operating budget on overtime last fiscal year — including $11 million to cover for absent workers. Chief Administrator Brian Shortsleeve has said the T has the highest wage rates for rail operators in the U.S.

“At the direction of the Fiscal and Management Control Board, a complete audit of overtime practices and usage is being conducted,” Pesaturo said. The audit’s findings will be made public, he said. “In the meantime, the MBTA will continue its laser-like focus on lowering operating expenses and implementing leave policies designed to reduce overtime spending.”