Instead of awkwardly trying to shove a stub CR platform in somewhere near Riverside, why not extend the Green Line less than 1/4 mile on tracks that are already there right now, and build another Route 128 Station on the Pike/128 interchange? There's no reason to assume that intercity rail won't want or doesn't benefit from having a Green Line connection, and the station could be used to dwell regionals for HSR overtakes or as a terminus for a Fairmount-esque line.
There are many reasons why that would not work.
1) Direct access to the D inbound < direct access to any other line. To get on any other branch you have to transfer at Kenmore or Copley and pull off the same number of transfers as you would going Red-Green from SS via Park or Orange-Green from BB via DTX or Haymarket. Nonstarter. We see this with the plethora of Orange Line-to-CR transfers...if they're going to transfer, they always opt for the high-density transfer spots at NS or BB, never Ruggles, Forest Hills, or Malden Ctr.
2) If Copley is your final destination, you're not transferring to the GL at all you're getting off the Amtrak or T choo-choo at BB and not transferring anywhere. If Kenmore/Fenway is your final destination, you're getting off the T choo-choo at Yawkey and not transferring anywhere. I am going to take a wild guess that the Amtrak Inland Regionals do not have a lot of built-in demand for Yawkey as a stop. Or if they do it's the one trip of the day that coincides with a Sox game...and if so they can happily add the extra stop to the schedule if they so choose.
3) The GL is not fast. On a delay-few morning, is it going to be a faster trip to Hynes from Riverside+1 you're proposing, or Red-Green from SS? Remember, if it's Kenmore/Fenway or Copley it doesn't matter because of BB and Yawkey. Maybe Hynes is the only one where it's a dilemma. Arlington...I bet Red-Green's faster. Boylston/Park?...Red from SS. Haymarket?...Orange from BB. NS?...Orange from BB. The only places the D is truly advantageous are stops on the D. Then take one look at the Blue Book boardings for the D vs. every single heavy rail stop and Central Subway stop and the rest is self-explanatory.
4) Train frequencies on the Worcester Line are nothing like they are on the NEC at Westwood/128 with Providence, Stoughton, Acela, and frequent Regionals all stopping there. Amtrak's only planning on 10 Inland Regionals + 2 Lake Shore Limiteds by 2025. Worcester certainly is never going to top Providence + Stoughton/(South Coast) frequencies. For the humongous infrastructure cost and all the things in reasons #1-3 that blunt its effectiveness, that's not much of a ceiling for this site compared to Westwood. And if Inland HSR comes to town, the whole point of it is that it will have WAY fewer stops than the current NEC does so it can only stop in the big cities, which probably relegates travelers getting off onto the 128 circuit to existing NEC service (or thru service if the N-S Link puts Anderson/Woburn on the Regionals map).
5) Access constraints. There's barely any developable land on Recreation Rd. next to the Worcester Line to plunk down substantial parking (see also: where's the TOD potential?). Golf course, Liberty Mutual HQ, boat landing, and a small maintenance yard crammed between a ramp and the river. Anywhere else makes existing Riverside closer...and existing Riverside is EASY to access from Pike or 128. Even poorer ped access, which is a big reason why the original CR station failed. Recreation Rd. is all high-speed ramps in this vicinity, and the only other access points are tiny residential cul de sacs who are most definitely not going to approve of people using their driveways as a kiss-and-ride. This is not an easily solvable problem, which is a big reason why current Riverside got built where it is.
They also
have officially studied a direct Worcester/Amtrak to GL connection...at North Station via the Grand Junction. NS a lot more central to the system than Riverside. You can reach GLX and the inbound branches more easily from there. It's a major terminal. They jumped the gun way too early on this with Tim Murray's mouth not doing him any favors with the City of Cambridge, but I think there's a pretty good chance of it happening in 10 years. And if it happens it removes most of the purported demand for the outer transfer. Except for that real robust Woodland-Longwood intercity market.
This is a solution in search of a problem. If a GL connection is needed for intercity riders, rehab the Grand Junction and do the officially-studied option. "Fairmounting" for strictly inside-128 rapid transit-like service does not require the end-of-line CR transfer there's little demand for...the existing Riverside stub will do. Force-fitting the superstation for several hundred mil more is not only a great way to ensure that they'll NEVER "Fairmount" to serve Allson, Newton, and the dense net of crisscrossing buses...but it might even perform worse--intercity modes and all--in that location vs. something simple and direct at the current Riverside serving all the new development at current Riverside.
