MBTA stuff:
Option 1: The MTA Shoreliners from TC have multiple MBTA reskins. However, seeing as you only have TS2012 I don't know if you will be able to use the reskins.
Option 2: On the DLS there are some Amtrak Horizon cars, with MBTA reskins available. Unfortunately, I do not recall where these reskins are available at, and these may require some asset fixing to work in 12. Plus, no Cab Car.
I will warn you that while either of the above are nice, neither are quite at the hi-res standard commonly found in newer content. As well, neither are 100% accurate to MBTA rolling stock. But I do have good news below!
Option 3: You may have noticed JR making posts that suggested that there is passenger coaches in the works. Not only are these sure to be great, but (and don't quote me on this, this is a guess off of older things JR has said, at that time about freight rolling stock) there is a chance they MAY be freeware. Downside here? They're still in production assuming they exist, and if they do, they still may be payware-but no matter the wait and price I'm sure they will be well worth it!
Good luck with the currently available stuff and lets hope for that JR content! As a Boston area person, I tend to keep an eye out for MBTA stuff (and passenger stuff in general), so this summer is gearing up to be great!