though I agree with @TheRatmeister , a lot of Washington is pretty narrow. The trains through Egleston must have been loud.)
I believe this is a photo of Egleston station, even with the black and white photo you can really see just how dark Washington St was, especially near stations.
For any elevated rail you should really consider the area under it to be lost space, in Paris and Berlin for example it's often totally inaccessible. On wider streets though it could be clawed back by using it for street parking.
Blue Hill Ave, Warren St, Columbia Road, Fellsway, Broadway (Somerville), Washington where the Orange el used to run (though I agree with @TheRatmeister , a lot of Washington is pretty narrow.
Of this list I'd strike Warren St, Columbia Rd, and Washington St. Even at Dover on upper Washington St where the street is about 80ft wide the station basically blocked out the sun. I think 100-110 is a more reasonable minimum width.
And so that leaves us with Melnea Cass, Revere Beach Pkwy, Fellsway/McGrath, Rutherford Ave, Broadway, and BHA. Here's my evaluation on those:
Melnea Cass: Would be great for connecting Andrew/Widdett Circle and Nubian.
Revere Beach Pkwy: The section from Broadway to Bell Circle could be useful, but it bypasses the densest parts of Revere. The rest could maybe be an orbital connection between Everett and Revere but again it bypasses the important parts of both and there also just isn't tons of demand for such a connection.
Fellsway/McGrath: Connects nicely to the Grand Junction but doesn't go anywhere particularly useful otherwise.
Rutherford + Broadway: Could be a good corridor between Medford or Everett and Downtown, but a tunneled continuation would be needed on both sides to make it useful, and if we're building a new subway line downtown I think sending it to Chelsea is a better choice, although Rutherford is well positioned to get to Everett.
BHA: What BHA doesn't really need is a fast line to downtown, that already exists and it's called the Fairmount Line. What BHA needs is a faster local service, capable of branching often and running many different services. (Aka, buses and/or trams)