MBTA workers are hoarding hundreds of thousands of sick days and using them to pad their pensions in a little-known union sweetheart deal that has taxpayers on the hook for at least ?$72 million, according to a new Pioneer Institute report.

T employees banked more than 380,000 unused sick days as of the end of 2014 — and the ?Pioneer Institute expects that number has grown, because a union perk allows workers to trade their paid sick leave for creditable time toward their pensions.

“There’s incredible incentive not to use sick days because it’s an annuity that keeps on giving throughout retirement,” said Mary Connaughton, director of government transparency at the conservative think tank. “With such generous perks, it’s no wonder financial problems continue to plague the T. The question is, how many more of them are out there that we don’t know about?”

In general, the more years a public employee works, the ?higher the pension payouts. ?A retiring T employee with 150 unused sick days can boost their pension by seven months, according to the 2013 MBTA Retirement Fund Annual ?Report.

Documents show 235 employees saved up at least 200 days of paid sick leave as of the end of 2014 and 11 workers had banked at least 300 sick days, according to T data obtained by the Pioneer Institute after a long-fought public records battle.

One T employee had more than 406 sick days at the time, data showed.

Connaughton said a T worker retiring with an average salary of $81,000 and 150 unused sick days would reap a nearly $29,000 pension pay bump over the first 25 years of retirement.

The perk is not enjoyed by most state workers, who typically receive a one-time payout upon retirement for 20 percent of unused sick time, ?Connaughton said.

T officials said an arbitrator’s decision granted the pension sweetener to the Boston Carmen’s Union in 1975, which paved the way for the benefit to later expand to all MBTA unions.

“This is another ?example of the burden of binding arbitration,” said MBTA Chief Administrator Brian Shortsleeve. “It certainly weakens management’s hands at the bargaining table.”

Gov. Charlie Baker last year sought to end binding arbitration at the T — a benefit not enjoyed by any other public unions — but state lawmakers defeated the proposal.

The new report comes as the MBTA looks to raise fares and T bosses crack down on widespread abuse of unpaid leave under the Family Medical Leave Act.

Shortsleeve said a new 2016 ?attendance policy requires staff to use paid sick days and vacation time concurrently with FMLA leave — rules the T failed to ?enforce in the past.

New MBTA data for the first three weeks of the year show overtime spending decreased and absenteeism fell for all workers except machinists. The number of dropped bus trips is also down.

“We don’t want to leave riders out in the cold,” Shortsleeve said.

“It appears that the new ?attendance policy is helping us to reduce absenteeism and reduce overtime,” he said. “It’s becoming an incentive for people to come to work. Any attempts to weaken concurrency and weaken our new attendance policies is something we would vigorously oppose.”