The proposals I saw for the BLX are all a bad joke requiring fantasy bridges across the Saugus River either above or below the General Edwards bridge, either of which would require substantial private land taking. The old ROW is just narrow gauge single track width and building a bridge there, just below the GE bridge would be ridiculously expensive and face massive opposition from the neighbourhood. Then it would be on the wrong side of the Lynnway and dead end with no hope of future expansion further north.
All in all no good is coming from that.
An even better idea than mine would be to electrify the entire route and replace the train sets with light rail vehicles.
The generally accepted alignment swings out to the intact 4-tracked ROW of the Eastern Route; the BRB&L alternative still exists on paper and as an historical footnote, but in all likelihood is DOA. The Eastern Route alternative doesn't require ED (or very little), re-uses intact Eastern width through Lynn station, allows for any deep-future North Shore ++ extension, but it does come with pricy wetlands mitigation and requires Red-Blue.
However, for the sake of conversation let's say that your original Orange branch idea is structurally feasible. Here's a major issue with an OL branch:
Wellington-State is the most consistently and severely congested segment of the entire rapid transit system in current circumstances. It's running up against a travel pattern shift for the near-North Shore: Everett, Somerville, Medford have all experienced substantial increases over the past 25 years in the number of their resident workers commuting to Boston, many well-within Orange's catchment area. Wellington-DTX is structurally over-capacity under normal operating conditions from ~ 8:15-9:00, with peak-of-peak regularly achieving load factors higher than the MBTA's crush capacity threshold. New rolling stock will offer marginal increases in frequency, more substantial improvements in reliability, but we're not talking world-altering differences. The OL
today has the most frequent headways (in theory, when it's running smoothly) in the rapid transit system, excluding the GL subways.
The only comparably structurally congested segments are the final legs on the Red branches and the Boylston Subway. And these vulnerable spots are shorter, usually no longer than one or two station-station intervals - the OL is dangerously congested along it's entire northern and downtown sections at peak. This is a function of ridership-heavy outer nodes and the major bus-terminal at Sullivan. By contrast, the Blue Lynn doesn't approach the MBTA service policy for congestion, save for Maverick-Aquarium on days with multiple delays.
Unless there is drastic improvement in schedule reliability, signal systems, an OL branch will enforce lower levels of service along one-half of the most vulnerable leg in the system. Even if Lynn-Everett-Chelsea service (really, just Everett; Chelsea and Lynn are, generally, bus-blue feeders) limits riderships along the northern stretch, it won't be enough and the Malden branch buckles under it's own weight. Everett is a massive market with substantial growth prospects; any half-way feasible rapid transit extension is going to drum up significant ridership, which is going to threaten Sullivan Square, an absolutely essential bus node if no boarders can get on a damn train. You can't reduce Sullivan Service and the express track isn't likely to be much a help here anyways since it only speed you up as far as CC if it were re-incorporated.
A branch OL will not, should not happen. Blue is the better option for Lynn, it's quicker, there's already an established travel pattern of Lynn-Wonderland transfers, and the line is not nearly as saturated as Orange. That leaves out Everett and Chelsea, yes, but there's no point expanding service even if it were feasible if the result threatens kills off Orange.
