Concrete Lechmere Viaduct needs expensive rehab (e.g. wrapping steel in epoxy and carbon fiber) in order to be future-proofed for longer, heavier, faster, more frequent trains (I think the design load is a 4-car Type 7/8/9 train, each way) even though we are likely only to see 3-car D's before the type 10s get here.
At the same time they need to turn the Old Lechmere site over to the Northpoint owners pursuant to the land swap that allowed them to build New Lechmere.
There is also some tearing down of the steel elevated and its replacement in elevated concrete that they couldnt do while Old Lechmere still needed its El approaches
"Trying to solve congestion by making roadways wider is like trying to solve obesity by buying bigger pants."--Charles Marohn