The stub is for a track to the new GLX LRV maintenance facility, although I think its been deleted in the final design.
Yep. That was intended to be a direct-to/from-Lechmere access point for the carhouse, as third access point accompanying the direct-to/from-Union and direct-to/from-Medford ramps. It was VE'd out for cost savings because it's not strictly necessary. The Medford ramp joins at a tri-track section with multiple crossovers right before East Somerville Station, and so it's also possible for a Lechmere deadhead to turn out onto Track 3, pause, and reverse onto the carhouse lead without blocking any Medford Branch revenue traffic. So they're going to pocket the savings and do it that way. Lechmere deadheads would be comparably rarer equipment shuffles than trains that simply run to Union/Tufts and deadhead back on the branches (i.e. the usual practice for shift changes), so they don't need optimal efficiency with a direct ramp from Lechmere to pull off those "other"-category moves.
The ramp stub is left for backfilling on future considerations. It's obviously a must-have if revenue service ever runs on the carhouse leads to Sullivan Square as either a +1 addition or the full on Mystic River-crossing Urban Ring NE quadrant to Chelsea/Logan. In that case the missing overpass would be added later, and the track configuration at the junctions would be substantially altered for the split in that direction. To illustrate:
Right now you've got a flying junction, and only one track is going to be laid on the loop ramp for flying outbound-to-Union off the Lechmere split. But you can see the first leg of the Viaduct before the ghost ramp is built at 2-track width...albeit squeezed out of emergency-egress side room because it's up on stilts in a constrained area and the signal system mid-junction is dense enough to stop all traffic fail-safe on the second berth to permit emergency egress on the other track. The ghost ramp is likewise 2-track width (and likewise without emergency egresses for same reason of other evacuation fail-safes being present). But the high loop to Union is much narrower single-track width...and
does have the side room for emergency evacuation (obviously with only 1 track you need some side means of evacuating a train).
If revenue service to Sullivan/etc. were ever enacted, you'd see the mainline junction at the very bottom of the photo changed from total-flying to partial-flying (for the Medford split) + partial-flat (for Union-or-Sullivan). That is: flat bi-directional junction switch and 2 inbound/outbound tracks on that first leg of ramp...then flat junction at the 'ghost' split where 1 OB-only Union track splits off from 2 Sullivan IB/OB tracks. Therefore everything is properly provisioned, and you just need to come back later to build a northern bridge abutment on the carhouse side and lift 1 prefab bridge deck into place spanning the tracks.
The cost savings from VE'ing that out probably weren't enormous, since we really aren't talking much missing infrastructure. But it was always somewhat of an ops mystery as to why this just *had* to be there in the base build because reversing off the Medford-side pocket always seemed plenty easy enough given how relatively few trains would be going in/out of service from the strictly Lechmere direction on a daily basis. It might've been a design hedge all those years the Brickbottom NIMBY's were fighting the carhouse and they were still iffy on the yard's exact parcel placement (and, thus, fluidity or lackthereof questions re: access dependencies from the branches).
