Nothing ongoing. It was supposed to be a long-term item for RIDOT boosted by the Amtrak service what-if's, but the Raimondo and McKee administrations have backpedaled away from literally any service increases south of Providence and buildout of anything Commuter Rail other than the Pawtucket infill so it's firmly on the backburner. Right now the NEC doesn't even have enough crossovers in the vicinity to attempt to pack bi-directional service denser on the current platform track, so they are stuck at not much greater than current service levels. And there's no plans to do the new northbound platform turnout required for fuller service to Wickford. Electrification of service there also requires additional capital spending, as the station overhang doesn't allow enough vertical clearance for P&W autoracks to travel on the wired tracks; P&W is forced to crawl at sub-10 MPH (so the cars don't smack the full-high) on the un-wired station track to get by. Electrification of the platform track requires installing a wide-swingout gauntlet track so the autoracks slot 'between' sets of wires and can travel at less-punitive speed restriction.
The project has been creeping its way through 30% design for a couple years. Per this year's update of the NEC Commission's Capital Investment Plan (p.20), the estimated project cost has now risen to a mind-boggling $360 million. Back in 2017, when RIDOT and Amtrak released a study on adding TF Green as a Northeast Regional stop, the capital costs were estimated at $90 million. So the cost quadrupled in 7 years' time.
When was the last time an extension was studied?
In 2011, but the study was never released.
There’s been a good amount of new residential development in Milford since 2011 (including high-density development), and there’s been some growth in Bellingham since then. There might not be enough ridership from Milford and especially Bellingham yet, but I’d think by 2035 and definitely by the 2040s there should be enough ridership to justify an extension to Milford. It’s also worth noting that Milford is very bikeable in addition to being walkable, with 2 branches of the Upper Charles trail converging downtown right near the tracks.
Trains to/from Milford could run semi-express, skipping Windsor Gardens, Norwood Depot, Islington, and Endicott to save a bit of time on the way into Boston. The 1997 Milford Commuter Rail Extension feasibility study (pgs. 11-14) looked into expressing between Walpole and Ruggles for Milford trains, but I don’t think it’s necessary or desirable for Milford trains to skip Norwood Central, Dedham Corporate Center, and Readville.
It would have to be rail-with-trail, as I doubt people in Milford, Holliston or Sherborn would be okay with losing the Upper Charles Trail to bring the train back. There might not be enough room in certain spots (specifically in Holliston) for both tracks and the trail.
An extension to Milford has been examined several times. There was the aforementioned MPO study in 1997, a brief examination in a 2003 Milford municipal planning report (which identified the preferred station site in Milford), the 2003 Program for Mass Transportation, supportive mentions in the 2010 and 2020 Bellingham master plan(s), and the 2011 MPO study (unreleased). 2011 was 13 (almost 14) years ago. It's definitely time for a new feasibility study.
Also, EMU service with full highs could reduce travel time to induce more commuters to move southward
Regional Rail
That became impossible when the NYNH&H abandoned West Medway-Bellingham Jct. in the 1930's. There was no freight west of Millis, the Millis freight jobs originated from the east instead of the west, and passenger service was truncated to West Medway so that short segment got shortsightedly chopped. Unfortunately it's impossible to mount now because the ROW is completely obliterated by newer housing developments, but it would've been a faster and more direct route to Milford and probably would've given Millis restoration a puncher's chance on ridership vs. cost if it were still there or at least landbanked.
There's an ownership gap past the last Conrail-era customer in Holliston after a Penn Central-era abandonment chopped the line, extinguished the operating charter, and reverted the property to the towns and abutters. It's only landbanked (still owned by CSX with the trail paying a small monthly rent) to a little past Holliston center. The trail, while contiguous, makes a couple small detours around breaks in ownership. So that line is probably never going to be a candidate for reactivation. Framingham-Holliston-Milford also already has bus service with MWRTA Route #6. The frequencies are very poor, but if there's a commuter audience for that corridor it'll come from upgrading the bus.
Mansfield's only 19 track miles from Providence, and the relative lack of highway congestion out that way makes the car trip via 495/95 to Providence much shorter than the one on 128/93 between 128 Station and Back Bay/South Station. So I much doubt there's an audience for this worth the time penalty to Regional schedules (remember: it's on a very long segment of 125+ MPH territory, so dropping a full station stop in there is going to add outsized schedule time). If there was calculable demand for it Amtrak would've trialed a Mansfield infill eons ago, as I doubt the T would have any objections. They haven't.
I’m thinking the purpose would be to avoid congestion from outlying suburbs going into Providence. Also, there was an Amtrak stop in the 70s, apparently. Have to think population growth in the meantime might make it more useful than then: Bristol County is about 30% more populous than in 1970.
Mansfield to Providence outbound on Commuter Rail is only 28 minutes and Mansfield to 128 inbound is only 19 minutes. With a projected 18% travel time reduction with EMU's that comes out to only 22 minutes to Providence and 15 minutes to 128 @ half-hourly service in a Regional Rail universe. The transfer ease is quite likely going to sop up a lot of the demand. You have to quantify what's left on the table with the traffic to Providence via 495/95 not being all that bad. Again...Amtrak probably has a good handle on what an infill would do vs. the schedule penalty of making another stop in high-speed territory, and it's probably not nearly good enough to be worth trying or else they would've tried it long ago.
The ROW is almost 40 feet narrower at the bridge than it is at the current platforms, so it's probably impossible to do even if you deleted the express track.