So, I have discussed this at length here, as an alternative to a second NSRL tube.
I'm not sure how you get 4-5 miles of tunnel. The ROW is clear until a bit north of Airport Station. It is 1.2 miles from Summer St to Eastie and .6 miles to the southern end of a RR station. That leaves a bit of...
OL to Nubian is the solution. Ridership on the southern half of the line drops off hard. Even with OLX to WRox, there should be sufficient space. A flying junction is nessary, but I think feasible
So, I know that both here and other forums, there are statements that Amtrak has plans to use the Front Yard for their own storage needs, but something has always bothered me.
No Amtrak NEC consists are short enough to fit there.