September will be costly. With the weeks-long shutdown of the Red Line's Braintree branch, commuters have been lining up and packing into yellow shuttle buses operated by A Yankee Line Inc. The Boston-based company is one of several vendors that the T has contracted with to provide shuttle service through the duration of its track improvement program.

"It's ridiculous how much money they spent on transportation and it never runs right," said Bob Cousy, 80, who complained recently that he had been waiting 40 minutes to catch a shuttle at Quincy Center Station.

A Yankee Line is being paid $6.45 million during the 24-day closure between JFK/Umass and Braintree stations, the biggest diversion so far, according to the agency. The T turns to Yankee Line the most during its scheduled shutdowns, and the overall contract is valued at $159 million -- a substantial increase from initial estimates of $30 million in 2021.

At the T's board of directors meeting in June, Yankee Line was singled out for being "uniquely successful" in meeting the T's requirements. As a result, the company has been awarded more than 90 percent of the T's business, increasing the cost of the contract.

Aside from Yankee Line, which has been paid or invoiced approximately $51.2 million for track improvement weekday diversions, the T has used Academy Bus and Paul Revere Transportation, along with A&A Metro for accessible van service.

Not including diversions that occurred solely on the weekend, there have been 12 shutdowns since October, when the Ashmont branch and Mattapan lines were closed for track work, according to the T. The cost of shuttle service for the first 11: $53.67 million (the agency has not received invoices for the latest).

The most expensive closure, by far, occurred when the Red Line was closed in July. Shuttle bus service cost the T nearly $14 million.

More diversions are planned through December, meaning: more shuttle bus service.

T general manager Phillip Eng said in an interview there's no way around it: "Giving the riders alternative travel is essential."

Although critics might bristle at the high price being paid for shuttle service given the T's funding crisis, Brian Kane, executive director of the MBTA Advisory Board, called it "a necessary cost."

"We can't leave people stranded," said Kane, a member of Governor Maura Healey's transportation financing task force. The agency itself is not equipped to operate their own buses during all of the diversions, he added, so employing the services of the private sector helps to avoid issues such as staff driver burnout.

Eng said the agency is on pace to have all speed restrictions cleared by the end of the year.

But unexpected future shutdowns are inevitable with any large transit service, and moving forward, Eng said, the T aims to be able to manage diversions differently. The agency is continuing to hire bus operators and "we hope to have a lot more of the shuttle buses being done in-house," he added.