FWIW the most current project universe document containing all the proposals that didn't make the FY17-21 CIP cut is available here as a large MS Excel spreadsheet:
http://www.massdot.state.ma.us/Informat ... Years.aspx" onclick="window.open(this.href);return false;
Easier to navigate if you set the filters on the "In_Out" header to "Out" to cut it down to just the projects omitted from the CIP, and then set the filters on the "Division" header to omit Aeronautics, highway, and RMV.
There's some pretty loopy proposals lingering on there due to past recommendations from the regional MPO's (Worcester Line extension to Springfield...seriously, Central Mass MPO???), so grade accordingly on a curve. The context to apply to this "CC-0122" proposal for a Cape Jct. wye track is that the other two Cape-region rail projects proposed but not included for the FY17-21 CIP both have to do with a full Middleboro-Buzzards Bay commuter rail extension: one proposal item # for the track improvements to support a full schedule (signalization & passing sidings...items above-and-beyond the Cape Main upgrades that did make the CIP cut), and one proposal item # for the BB station improvements (full length full-high platform, parking enhancements, etc.). So the Cape Jct. wye track appears to be mutually supporting of the BB commuter rail extension, and not something proposed by itself in a vacuum. That's important.
If I had to guess why it was mutually supporting: having commuter trains idle and reverse direction on the platform @ 10 round trips per day places an increased premium on keeping on-Cape trains from needing to cross the bridge and go through the BB platform in order to terminate (i.e. dinner train) or reverse direction (i.e. Falmouth trash trains). The layover yard for full CR service would continue to be at Middleboro with trains not immediately reversing at BB deadheading back to M'boro (the layover was built oversize from Day 1 with a future BB extension in mind). The only storage at BB-proper will be a controlled siding for passing and short-term idling to allow an MBTA train that's just dumped its outbound passengers to scoot out of the way for an imminent bridge opening. Eliminates outright conflicts, but CCCR and Mass Coastal are still a lot more pinched for slots than they were before. For the dinner trains that may mean needing alternate accommodations for a westbound terminus if they ever want to increase their schedules, since BB platform slots will be harder to come by and probably won't allow any more between commuter slots than what they run today. So a wye leg that changes the trip itinerary to "ringing the shore" Hyannis-N. Falmouth may be the business proposition they need to reorient to for future growth if they're capped by MBTA meets on slots across the bridge to BB Station. For the trash train it's simply an operational convenience to not have to cross the bridge at all to get to Otis Transfer, and to be able to block and can freights entirely on the Cape side of the bridge for purposes of precision-timing their shots to Rochester in between MBTA schedules. Taken together, there's probably enough give-and-take to have MassDOT convinced enough that the (relatively inexpensive) wye build is a necessary counterbalance to CCCR/MC for impacts of the commuter rail extension.
The T's fiscal board just greenlit the Foxboro commuter rail pilot today, which ensures--via a now locked-down early-'19 service start and 12-month evaluation period--that that new limited service has very high odds of graduating to a 'permanent' full schedule with a second shot of later upgrades fulfilling the goals of the full-build Foxboro Feasibility Study (i.e. a full 16 round-trip all-day schedule instead of just a couple rush hour extras). Buzzards Bay, because of its inclusion in the '13 transpo bill and Town of Bourne formally voting itself into the MBTA district, under normal circumstances would've been par (or even a smidge higher) with Foxboro on priority level for a pilot. The only reason the state couldn't consider one now for them was because of all those FY17-21 programmed track upgrades to the Cape Main, which are going to take place during the Cape Flyer offseasons. That includes replacement of the Cohasset Narrows 2-track rail bridge just west of BB station, so there's enough construction interruptions packed into those fiscal years to make a Foxboro-like trial impractical for the rest 4 years. HOWEVER, Bourne's vote to join the T district and past promises made are going to make a BB pilot very high priority for the first fiscal years after construction...and now that Foxboro has earned their pilot there's too much precedent in motion for the T to deny Bourne et al. Now throw in the odds of the Foxboro pilot becoming enough success that a perma-build gets funded?. . .
. . .yeah, you could very easily see the same sequence play out: a 12-month trial of Buzzards Bay commuter service in 2021-22, followed by this trio of "PROPOSED" Cape rail line items on the spreadsheet quickly being graduated onto the next CIP revision post-FY21. Including that Cape Jct. wye leg if it truly is joined at the hip with the other line items re: commuter service to BB as necessary traffic mitigation for CCCR/MC. I'd consider the odds way better of this one graduating to future funding than some of the other "loopy" passenger rail residue littering this spreadsheet.