This is incredibly frustrating, but it is also still quite confusing. It does not make sense for it to be structural, since they took the whole thing offline for a year to refurb the structure. The appearance was the thing being historically preserved; no one cares if the the performance aspect of the structure is original or not, such as if that meant they needed to install supplemental steel reinforcements underneath it (or whatever it would have taken), and they certainly had the time to do that if that was actually necessary. So I have a hard time believing this is structural. In various places I had read reasons being about both needing to obtain special ties on the old movable drawbridge span portion of this, as well as about catenary/ overhead power line issues. To date I do not believe the T has released transparent, comprehensive information about this and I really wish they would - this slow zone compromises the efficacy and overall promise of GLX, a major public project, so I feel the ridership is owed an update.