^^^^
Rifleman, I'll throw in the other Universities in Boston/Cambridge and the immediate area as well, as being the constant economic engine that basically powers the immediate area. But in terms of major investments in infrastructure, it's gotta be the Big Dig as #1 and I'd have to agree with CSTH with the Harbor Clean-up as second which didn't even occur to me. Frankly, if we were talking about the metro area, I'd almost have to say that the construction of 128 was the transformational (to use CSTH's perfect term) project that started it all!
Atlantaden -- all of the above were important contributors to the local prosperity in their own time and in their own way and you can certainly keep winding backward and picking other important progenitors
However, what seemed like one giant mistake at the time has turned into the most important positive contributor
When Cambridge cleared the deck for NASA's Electronics Research Center and which promptly fizzled after "regime change" -- the big open area behind MIT ended-up as the location, location location for today's Kendall Sq. Renaissance
Had it stayed decaying industrial it would have been developed piecemeal
Had it become the site for the new Cambridge City Hall complex or a huge complex of low income housing -- we wouldn't be talking about the boom
But it stayed relatively fallow through abortive wana-booms [i.e. AI Alley] and then finally Bio -- powered by the computing of the Human Genome lit off like the proverbial fireworks finale -- the rest is as they say history
Where else but Kendall could you site an up-and-coming contender university [circa 1915] on recently made land next to a thriving industrial district and then magically 70 years later have a clean slate upon which to site the Hub of Innovation
Could some things have been done better if there was prescience circa 1980 when everything seemed so bleak -- of course
For instance the Red and Green Lines should have been connected along what is now First St on through Binney & Galileo to Broadway & Prospect and ending at Central with surface stops a few thousand feet apart [comparable to the B Line in the middle of BU]