Very nice map, George_Apley, am glad someone took the time to sketch out what F-Line outlined in the other thread.
With respect to Red Line via Route 2 -- I think the best approach here would just be an elevated no-stops express straight out to a Park-n-Ride at 128. Handle Arlington and Lexington commuters with a separate tactic (see below), and use the Red Line solely for clearing out suburban/exurban commuters from Route 2. Combine the Park-n-Ride with dedicated bus feeder service to the office parks (and/or something like the LRT George_Apley has outlined under "Other Transit Concepts").
No-stops express isn't going to float a build. Especially when the actual 128 station appears to be the most problematic part of this routing. It's safe to say nothing is ever going to get greenlit on rapid transit that goes >2 miles between stops. It's antithetical to the mode's advantages, and a seeming admission that the corridor in-question isn't up-to-snuff on stop selection (which may indeed be true here).
The best corridor for Arlington and Lexington commuters is the Minuteman path. Hits the local town centers along with the various residential outgrowths. It would be the perfect path for the Red Line, if it didn't require tunneling and/or destroying the bikepath to run largely at-grade HRT. I think the best solution is a LRT line along the Minuteman. It'd be tempting to hook it in to a larger Green Line network, but I think it would be better to short-turn most services at Alewife or Porter -- maybe run some peak services through. You would need to keep service very firm and consistent, especially going through Arlington, in order to replace and improve the 77.
The Red Line is already angled into Arlington, and would remain in shallow tunnel out to Arlington High School. So there's no question that's the mode choice for Arlington Heights. LRT won't be a choice...just continuation of Red into Lexington. And since the primary opposition to Red into Lexington is the cannibalization of the most scenic part of the Minuteman into a tightly-packed rail-with-trail with no separation, it's not going to matter if it's Red or Green. The problem is still the same.
Now, I think the regional transpo network is going to be missing a limb if we don't eventually get to Hanscom...especially when Purple Line Urban Rail will be reaching 128 in all directions and rapid transit extensions like Orange-Reading doing the same. So we're eventually going to have to come to grips with this. But it's going to be a big ordeal to get through Lexington, while getting to Arlington Heights is not. Going to have to be broken into two separately-mounted phases with an indefinite interim period where Arlington Heights is being used as a springboard for bus connections out to 128.
This actually dovetails with my other thought on the "Red-X" proposal. While I understand the appeal of routing the new branch to Woburn, I have a slight preference toward sending it to Waltham.
For one thing, I think that an Indigo/RER service is a better alternative for the Lowell Line -- the city of Lowell itself merits high frequency service, particularly since it's currently only a 45-minute trip (and would likely be a bit less post-electrification) and thus is one of the shortest end-to-end trips on the commuter rail. Layer in Haverhill service south of Wilmington, plus some short turns at Woburn, and you have a nice level of service. A full Red Line extension up to Anderson would require adding tracks that I would just as soon devote to mainline rail.
By contrast, on the Fitchburg Line, after 128, there's much less potential for RER. Lowell is 22 miles from Boston; 22 miles out the Fitchburg Line is... Acton. There's no anchor for the service until nearly 40 miles out in Leominster. There's a steep drop-off in ROI for RERification on the Fitchburg after 128, while the ROI is much steadier on the Lowell.
There's also a bit of a density drop-off between Woburn and Medford, though to be fair that is also mirrored a bit in Belmont. But the Fitchburg Line is able to serve downtown Waltham, while the Lowell Line famously misses its town centers including Woburn.
The fault with this routing scheme is that Fitchburg has all the tough grade crossings, while Lowell does not. We actually discussed this a few pages ago re: sending Green to Watertown, but the Sherman St. grade crossing being all kinds of awful. The only feasible solution seen in that discussion was separating Green in a steep duck-under but leaving Fitchburg at-grade. While most of the other crossings (Blanchard, Beaver, Elm, Moody, and South) can feasibly be eliminated on HRT, some of them are going to be roller-coaster rides up/down inclines to get out of the way of adjacent bridges. "Technically" feasible, yes...but in an awfully fugly way that's not going to do much for ride comfort. LRT would not have those issues, especially in cases where choosing to engage the crossing may indeed be better than standees hanging on for dear life at another kamikaze dip.
Lowell has a straightforward fix for the West Medford crossing pair once critical mass makes the high cost palatable, and from there it's completely grade separated out to North Chelmsford. There are no special construction considerations out there for HRT, just as Fitchburg offers no special construction considerations for LRT. The choice of modes needs to be flipped for this reason, because the extra hardships to grade-separate the Fitchburg route solely for choice of HRT are not worth the sharply increased price tag.
As for density...there's plenty of it at Winchester Ctr., especially if bus coverage were amplified accordingly. The cavity around Mystic Lakes to Wedgemere isn't indicative of the whole corridor. Unfortunately nothing is going to bring the Woburn Branch back, as it's physically obliterated by new construction. Montvale Ave., however, is as close a walking route as you'll get to downtown, has the equal proximity to Stoneham, has the I-93 access, and has the existing buses plus lots of potential for more (esp. plugging the coverage hole in Stoneham). I don't think the density difference is so head-and-shoulders different as to change the whole mode selection and the need to engage all those crossing expenses.
Keep in mind as well, each leg of the "X" is going to have 3-minute peak headways. That's arguably excessive for a lot of outer stops. What is now the 3-track Alewife yard was built with Lexington in mind as a means of short-turning trains on the center pocket track to throttle down frequencies to the suburbs. Quincy Center's pocket can serve much the same function. So, I suppose, would Codman Yard at Ashmont if service were extended to Mattapan or Hyde Park/Dedham. If there's a dropoff point where headway throttle-down is needed, just do the same at Route 16 or somewhere. It would be entirely consistent with how a suburban-expanded Red Line was supposed to work.
But at a more abstract level, I lean toward a Waltham routing because it allows the two Red branches to intersect again. This "new" Red Line branch would, on the south side, serve as a de facto express service counterpart to the existing service. (Especially if Broadway was eliminated, which I think is worth considering.) Transfers available at JFK/UMass and South Station, and a Waltham route would enable a third transfer node at Porter or Alewife. Between Porter and North Station, there would be at most -- what? -- two intermediate stops? (And again, especially if Community College were eliminated.) So you'd have express service between Porter and Downtown, and between JFK-UMass and Downtown.
Finally, a Waltham routing opens the door to a junction somewhere around Alewife, allowing Route 2/Arlington trains to either travel via North Station (express) or Park (local), and likewise allowing Waltham trains to either travel via North Station (express) or Park (local). This would mirror the same options that would be afforded at JFK-UMass, which opens the door to a range of routing options.
None of this is feasible at all. The Fitchburg ROW is much too far away from the Alewife tunnel. 1500 ft. is roughly closest approach, and on either side of the station the approaches are 2000 ft. away. It's not clear you could even get between the Fitchburg ROW and Alewife Station for a union stop because of all the housing developments and wetlands in-between. And there would be no way to get back on-alignment because the existing tunnel turns steeply into Arlington and any number of CambridgePark buildings would have to get blown up to continue anywhere west.
This isn't feasible at all, and the destruction it would entail in North Cambridge is absolutely untenable. Concession to practicality is necessary here: the legs of the "X" will not ever re-intertwine after diverging.
Having an express service also makes it a bit easier to sell an LRT feeder line for Arlington and Lexington. Yes, it's not a two-seat ride, but once you hit Alewife, you can transfer and be in downtown in 10 minutes.
Moot point because, as above, re-engaging the "X" is physically impossible and Red is the one and only mode capable of going to Arlington due to the infrastructure already present past the Arlington town line from Alewife.