I found this on another site (a few posts down it says you can carry on the MBTA). I do believe this is true. I have read other places before that you can carry on there. I am not a lawyer and couldn't find it searching the MBTA website, however, in my opinion you will be good to do. The person that wrote this on that site is very knowledgable. You should be good to go. Hope this helps, I will keep trying to find more.