Well they were always the Transit Police, but never used the title, they prefered MBTA or T Police since they started. But rumor is (gee how many of those floating around) is that they are not merging with anyone, but will fall under the Office of Public safety which many agencies do now, not just Mass Police agencies. And the rumor further states that they will take over these regional transit systems like BART in Brockton and the one in Lowell amd Merrimack. Again, that's what has been said along with a hundred other plans.
NOTE: They should have taken the last NY Transit cruiser design before merging with NYPD. That looked sharp and had a nice clean look.