An audacious plan for expansion of rapid transit in Boston by electrifying the existing commuter rail lines. Bears quite a few similarities to this fantastic diagram from 1945, which also advocated rapid transit along railroad rights-of-way.
Map from a 1972 proposal for the Boston subway lines to take over the commuter rail lines far out into suburbia, with the Green Line reaching Newburyport, the Green Line reaching Haverhill, the Red Line Fitchburg and Ayer, and the Orange Line almost to Providence, amongst other branches. (via twitter user @sandypsj)