I decided to visualize what would have been possible if some of the money spent on the Big Dig was put into transit
You wouldn't have gotten all $14b into transit: Eliminating the Fitzgerald elevated was going to be costly no matter how you did it. Even torn down and replaced by a surface boulevard, you were going to have expensive transitions at both ends:
I-93
- tear down Fitzgerald elevated ($1b?)
- new Mystic (Zakim) Bridge with boulevard-to-I-93/MA-99/MA-28 transition ($1b)
- new South Station boulevard-to-Southeast Exway transition ($1b)
- new Pike-to-93 transition ($1b)
- Other surface improvements @ South Bay and MA 99 Washington Ave Bridge Rutherford Ave (would still be crowded) ($1b)
And you've probably had to accelerate:
- 128 widening ($1b)
- new 95/93 junction in Canton ($1b)
- new 128/95/93 junction in Woburn ($1b)
So that's $8b just to create a surface boulevard and to divert "interstate cargo" around 128
I-90
- extend to airport via TWT to keep airport traffic off surface boulevard
(or provide better Airport train facilities, perhaps "under the Control Tower")
Without the Tip/Liberty/Ted, you were going to spend "East Side Access" size budgets on enormous rail caverns instead of enormous highway caverns:
NSRL ($4b built solo with Fitz teardown)
Central Station ($2b)
Airport Rail Link ($2b)
Red-Blue connector ($.5b)
So that's about $14b of trains in the core and highway bypass before any linear transit extensions. Am I off by billions? Surely (total above is $16.5).
But there isn't a huge pot of cash for BL to Lynn or GL to Medford and outer stuff. And you're probably going to have to buy a $1b worth of electric trains and rolling stock to make the core stuff work.