The MBTA announced
two further phases of the double-tracking project today:
Phase 2, which will begin construction after Phase 1 is complete this December, will add "another 3.5 miles of double track west from Norfolk Station to Frank Street, which is just east of Franklin Station". I assume that means Frank interlocking, as there is no Frank Street. 3.5 miles from there is west of Norfolk station, so I guess they're avoiding all station work in that phase. But it'll allow headways to drop from 45 to 35 minutes. (Where are they getting the extra rolling stock?)
Phase 3, which will begin design in 2020, will complete double-tracking on the "entire line between South Station in Boston and Franklin Station". I take that to include the 4-mile gap from Walpole to Norwood, the small gap at Norwood, and one or both sides at Readville. That would involve new platforms (and thus very likely ADA modifications) at Windsor Gardens and Walpole, and additional/modified platforms at Norfolk and Readville. Plimptonville will probably bite the dust.