According to research set to be released Thursday by the Pioneer Institute, the MBTA pension fund would have $260 million in additional assets if it had been invested with the Massachusetts state fund from 2005 through 2014, reflecting the state’s higher returns and lower expenses.

The step is part of a push by administration and Massachusetts Bay Transportation Authority officials to improve investment performance and governance at the $1.5 billion retirement fund for transit workers.

The Baker administration is exploring ways to press the MBTA pension fund to move some or all of its assets into the larger state pension fund, according to people briefed on the developments.


“This is a huge difference,’’ said Iliya Atanasov, senior fellow for finance at Boston-based Pioneer, a think tank that is often critical of the T. In addition, his report says, the transit workers’ pension expenses are significantly higher than the state’s, which is run by Pension Reserves Investment Management.

For the 15 years ended in December, the MBTA pension fund has produced an annualized return of 4.6 percent, compared with 6.3 percent for the $60.4 billion pension fund for Massachusetts teachers and other public workers.

The 15-year median return for all US public pensions with assets over $1 billion is 5.8 percent, according to the Wilshire Trust Universe Comparison Service.

Steve Crawford, a spokesman for the MBTA pension fund, declined to comment.

Pressure has been mounting on the fund to change its approach. The governor last week signed a measure that makes the pension fund’s records public, a momentous change at an organization that receives tens of millions of dollars in public funding but operates as a private trust.

On Monday, the fund’s executive director for the past decade, Michael Mulhern, told the T pension fund’s six-member board that he will resign in August. He said in a letter that he believed the time was right to move on, after overseeing an expanded annual report for 2014 and a consulting review that refuted criticism of the fund’s accounting methods and investment performance.


Spokesmen for the governor and the MBTA declined to comment on plans to press the authority to invest with PRIM.

Legally, the maneuver could be tricky, because of the fund’s private-trust status.

Politically, it could be challenging as well. For many people in the transit system, there is a deep distrust of allowing their plan to be managed by the state, dating back to a time years ago when the T’s plan was fully funded and the state system was doing poorly.

But the MBTA’s chief administrator, Brian Shortsleeve, is a Baker appointee who is working to narrow the transit system’s budget gap and has been examining everything from its debt structure to the pension’s liabilities.

On Mulhern’s watch, the T pension fund has seen two hedge fund investments fail. One, for $25 million, was sold to the fund by Mulhern’s predecessor, Karl White, who had gone to work for a New York hedge fund, Fletcher Asset Management.

Under normal ethics rules, White could not have pitched the investment for at least a year after quitting, but the T pension is not subject to those rules.

Since the 2008 financial crisis, the T pension fund’s reported investment results have been almost identical to PRIM’s. Looking at the aftermath, from 2009 through 2015, both have had annualized returns of about 10 percent. After fees, the T would lag PRIM, according to the Pioneer report.


The state Senate’s minority leader, Bruce Tarr, this week told the Globe the T pension board does not have to hire PRIM to manage its money, but should consider that option.

Michael Heffernan, a Baker appointee to both the state pension board and the T pension fund, on Wednesday announced he was leaving the PRIM board to focus on his job as state revenue commissioner.

A former chief of the state pension, James Hearty, is the governor’s nominee to replace Heffernan on the state pension board, according to people informed of the selection but not permitted to discuss it publicly.

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