Updated MBTA photo policy - dispenses with requirement to show ID if asked, but see the section about not taking photos of "restricted areas," basically inside normally locked doors and the like where only workers go. MBTA Transit Police tweeted this means:

If you are in a public area and can see it, you can photograph it. Stay behind the yellow line!

The policy might be worth printing out if you do take photos with something other than a cellphone, just in case you run across a T worker who objects to you clicking, since they still seem to be out there: