As originally designed, it would have followed the right of way of the former Penn Central/New Haven Railroad mainline (current Amtrak Northeast Corridor) running from Readville, north through Roslindale, Forest Hills and Jamaica Plain, where it would have met the also-cancelled I-695 (Inner Belt Expressway).

[1] Another highway, the four-lane South End Bypass, was proposed to run along the railroad corridor between I-695 in Roxbury and I-90 near Back Bay.

[3]: 31 Between the Boston terminal in Park Square and Forest Hills, the B&P mainline was at grade except for Hogg's Bridge, which carried Centre Street over the railroad.

[7] The small local cost provided towns incentive to petition for crossing eliminations to prevent public thoroughfares from being blocked by trains and to avoid deadly collisions.

In July 1890, local politicians began planning for a grade crossing elimination project on the busy B&P mainline through Roxbury and Jamaica Plain.

[8] The project started in 1948 with Massachusetts Public Works director William F. Callahan's Master Highway Plan for Metropolitan Boston, went through several adjustments and then was terminated in 1972 by Governor Francis Sargent, following popular pressure.

Fred Salvucci described Sargent's decision to cancel the expressway before the Interstate Highway Option transfer as "It was like he jumped off the top of the Prudential Center and hoped that Alan Altshuler would have a net woven to catch him by the time he got to the bottom.

[11] In 1973, the state planned to build a 300-space parking garage – to be served by express buses to Boston, Providence, and Logan Airport – on the stub end of I-95 just inside Route 128.

The Southwest Corridor Park, maintained by the state DCR, has become a vibrant space for pedestrians, bicyclists, dog-walkers, amateur sports leagues, and community gardeners.