The Red Line Extension marked the end of the era of mindless highway expansion. It was born in the ashes of the Inner Belt Project, which had sought to extend highway viaducts through the middle of Cambridge and the Fenway. After a public outcry and extensive planning discussions, highway funds were redirected toward the enhancement and extension of Red Line, to Quincy and Braintree on the south side, and from Harvard Square to Alewife on the northside.
The Red Line's NW Extension included 4 new stations and 3.2 miles of twin tunnels. Much of the project, including the tunnels from Harvard to Davis Square, and Porter Square station, was mined from below, almost invisibly.
This project is in many ways a model for the North South Rail Link. Construction was completed with minimal surface disruption, on schedule and within the projected budget, at a cost of about $1.6B in current dollars, far less than the benefits it has unlocked for Cambridge, Somerville and the western suburbs.