The Southwest corridor or Southwest Expressway was a project designed to bring a four-lane highway into the City of Boston from a direction southwesterly of downtown. It was supposed to connect with the Interstate 95 highway at the Yankee Division highway (MA 128).

The project started in 1948 with William F. Callahan's Master Highway Plan for Metropolitan Boston, went through several adjustments and then was killed by popular pressure. Having been witness to recent housing clearances for the Interstate 93 expressway as well as similar projects in New York City and other cities, the population of the affected area was largely unwilling to repeat similar costs for another expressway.

The corridor was later recycled into the new route for the MBTA's Orange line and Amtrak's Northeast Corridor with much of the surface area being developed as a 52 acre (210,000 m²) linear park.

[Southwest Expressway - Unbuilt (http://www.bostonroads.com/roads/southwest/)] - A very comprehensive overview