Sullivan to Medford...where? As a Commuter Rail shuttle?
The only ROW options I can see out of Sullivan that hit Medford are either north alongside the Orange Line or south from Yard 21, through the wye and the tracks over to the Lowell Line (if they haven't been severed by the GLX by this point, and then up that way. Up the Orange Line/Haverhill way you could theoretically run shuttles on the Haverhill single track if you wanted to make that chokepoint more of an operational headache, but the only place you can go on that alignment is the Medford Branch, which dead-ends
here at the cold-storage warehouse, with the rest of the ex-Medford Branch ROW obliterated. Pan Am's trackage maps still show the Medford Branch (what's left of it) as active, so you're stuck with CR until and unless you can pry it away from them (and for all we know CSX might actually be interested in serving customers on that little spur). I suppose in theory if it came off the national network you could make it an Orange spur, though crossing the main inbound track to get over there for outbound shuttles is just asking for trouble, and you can't really shiv a shuttle track in there because the Wellington carhouse expansion took up a bunch of space where there used to be a track for storage so there's very little room to work with without impacting the yard.
Going the other way gets more Somerville than Medford at first, but it also makes very little sense. The simpler solution would be to run Green Line tracks from Sullivan (eating Yard 21) down past the new maintenance facility, where they could run to (if you completed the flyover junction, as I understand it) either GLX branch, the Lechmere Viaduct or the Grand Junction (if you built that as GLX a-la the Green Line Reconfiguration thread). Even if what you'd more likely
get is swapping trains on the Lechmere platform to go Sullivan-Lechmere-Medford, it's considerably more practical to implement than a whole new shuttle.
