PraiseX6 wrote:(I know I'm going to get bagged for this but) - This fact I just learned today - the bridge after NS to Lechmere (I'm not sure if this includes before / after science park) - Can only allow 3 trains at once due to a weight restriction. This is ALSO a huge factor in deciding what to do.That shouldn't be much of an issue. If the Viaduct could handle 3 or more feeder lines out of Lechmere causing traffic backups in the old days, and 3-car trains at isolated times with modern equipment...then 2 extensions with much lighter ridership than the westside branches shouldn't be a problem. You'd be extending the E one stop and adding another line (be it Brattle Loop-ing or another line. 2 lines have terminated there at various times in the past, so there's nothing inherently new about tapping the extra capacity on the northside end.
The Viaduct is a multi-segment span and trains would not be riding bumper-to-bumper across the entire length, and it does already get quite a bit of traffic during Garden events when they do sometimes run Lechmere-Brattle Loop cars to disperse the crowds in addition to higher-headway E's. The Viaduct is also long-overdue for structural renovations since they disappointingly skipped all but the most cosmetic concrete fixes when the line was closed for North Station relocation. They will have to do something a bit more substantial to the crumbling concrete on the underside of the structure when the extensions open. I would actually think multiple-train vibrations pose a bigger challenge to the old concrete than weight...it's a hefty structure, but it needs a thorough resurfacing before chunks start falling off the underside.
