I enjoyed the well-thought-out plan for this Yellow Line. It is going to have to be done underground as Sloan said, but my concern is where the line will run underneath the sunken I-93 highway that resulted from the Big Dig. Would this be the nail in the coffin for a North-South rail link that could carry Amtrak and/or MBTA commuter trains between the two downtown stations? Or would the Yellow Line run over or under a rail link in a bilevel tunnel? Also, I don't think the narrative addressed a yard/storage facility for this line. Especially if it's going to be totally underground, more than likely the yard will be outdoors someplace along the line. Since it will run through densely populated areas, where would they put such a yard? Perhaps instead of the Yellow Line running parallel with the Red Line in the South Station vicinity, the two could interline and run together, thus using the existing South Station infrastructure and allowing Yellow Line trains to access the Red Line's yard in South Boston.
Another concern I have is how this would compete for funding with the planned Green Line extensions, as well as the Indigo Line that would also run through many underserved parts of the Boston area as well as "transitize" the existing commuter rail Fairmount Line.