BOSTON — The cost of bringing Boston’s transit system into overall sound working order has climbed again and is now estimated at more than $7.3 billion, transportation officials said Monday.

The new number represents an increase of more than $650 million in the state of good repair backlog over another estimate earlier this year – a figure which itself had more than doubled in five years.

Massachusetts Bay Transportation Authority officials say they considered more assets in their system this year, leading to a jump of $650 million over last year for the cost to upgrade the system and make sure equipment is in working order. 2015 Associated Press file

The backlog is defined as the total amount it would cost to bring the Massachusetts Bay Transportation Authority’s vehicles and equipment that are in disrepair or operating beyond their customary lifespan up to ideal working order. Transit officials stressed that the backlog is not a reflection of the safety of the system or its ability to function on a daily basis.

“State of good repair is ultimately about the assets that we already have and making sure that we keep those assets so they are able to serve their intended purpose,” said Secretary of Transportation Stephanie Pollack.

The MBTA’s aging infrastructure was exposed during a severe stretch of winter weather earlier this year that resulted in massive equipment breakdowns, canceled trains and stranded passengers.

Erasing the state of good repair backlog over the next 25 years would require an annual capital investment of $765 million, not even accounting for inflation, officials told the MBTA’s Fiscal and Management Control Board. The T has dedicated $610 million of an approximately $1 billion capital budget for the current fiscal year to good repair issues.

The backlog was estimated at $3.1 billion in 2009. Officials said the $4.2 billion increase since that time reflects a more comprehensive accounting of the system’s assets and not a sudden or dramatic deterioration of vehicles and other equipment. The most recent figure was based on an examination of 250,000 individual assets, while in 2009 only about 95,000 assets were reviewed, officials said.

Gov. Charlie Baker called for creation of the five-member control board after a task force he appointed found the MBTA in “severe financial distress” and with no viable maintenance plan. The board, which will oversee the transit system for at least three years, was approved by the Legislature as part of the state budget.

The board has promised to establish five- and 20-year capital plans to address the backlog.

Also Monday, control board chairman Joseph Aiello called for the hiring of an independent consultant to investigate projected cost overruns that cloud the future of a long-delayed project to extend the MBTA’s Green Line to Somerville and Medford. The board was warned last week that the price tag for the 4.7 mile extension could jump by as much as $1 billion.

Board member Brian Lang said he would support bringing in a consultant and expressed frustration over the new cost projection after the proposal had been vetted by state and federal agencies.

“It’s mind-blowing to me that so many could have looked at this and said, ‘this makes sense, the numbers line up,’ and for it to be so far off,” said Lang.

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