The new figure is up $129 million for 2015, based on the pension fund’s new actuarial approach, which eases the impact of bad years. Without using that method, the unfunded liability goes up to nearly $1.1 billion, according to the T.

Just four months after the last adjustment, the gap between future retiree benefits and the money available to pay them is expected to rise to $944 million, according to estimates the T’s chief administrator plans to present at a public meeting Monday.

The MBTA retirement system’s unfunded pension obligation is about to increase again.

The Massachusetts Bay Transportation Authority is still awaiting the pension fund’s financial valuations for last year. But if the estimates prove accurate, the pension fund, with $1.5 billion in assets and $2.6 billion in future obligations, is now just 58 percent funded.


In the retirement world, falling below 60 percent often brings greater pressure to improve investment performance — or cut costs.

“This is the time for us to take a hard look at the sustainability of our pension [fund] at current benefit levels and engage in a collaborative discussion with all the stakeholders,’’ said Brian Shortsleeve, the T’s administrator.

Shortsleeve and the Baker administration are trying to persuade the T’s pension board to consider moving the assets into the state pension fund. That fund is far larger, at $60.4 billion, and has the benefit of scale when it comes to expenses and securing top managers.

But the T pension fund is organized as a private trust, not as a public entity, despite receiving more than $70 million a year in public funding. So it could require action by the Legislature, as well as approval by the Boston Carmen’s Union, Local 589, to move money to the state’s Pension Reserves Investment Management.

According to the T’s calculations, which will be made public Monday at a fiscal control board meeting, the MBTA pension fund’s assets declined 7 percent from 2005 through 2014, while the assets of the state teachers’ pension system -- managed by PRIM -- have increased 25 percent.


A spokesman for the MBTA pension fund was unavailable for comment Friday.

The T pension fund faces a difficult challenge in its sheer numbers: Unlike the state’s large public retirement funds, the T has more retirees than active employees. There are 5,868 people working for the transit authority, versus 6,478 retired workers receiving benefits, according to the report.

The pension fund pays about $187 million in benefits annually and must produce an annual return of at least 5.9 percent to cover that, Shortsleeve said.

Michael Mulhern, the pension fund’s executive director, recently announced he would resign in August, after a decade on the job.

The pension fund for transit workers has come under sharp criticism over the past year. A group of financial analysts led by Wall Street whistle-blower Harry Markopolos and Boston University finance professor Mark Williams last June alleged in a report that the pension fund was overstating its financial health by as much as $470 million.

The pension board responded by hiring a consulting firm to conduct a financial review of the fund and refuted the allegations.

Beth Healy can be reached at beth.healy@globe.com. Follow her on Twitter @HealyBeth.