At this point, this features almost as a given to every Future MBTA map I sketch. It's grade-separated until Arlington St/Nichols Ave in East Watertown, and Arsenal is wide enough that you could probably swing dedicated lanes. Certainly at least to East Watertown, it's a no-brainer.Easier said than done, but let's explore...
- The only practical way to get to Watertown is by extending GLX out of Union to Porter and from there reactivating the Watertown Branch ROW, which splits off of Fitchburg at Fresh Pond Mall. At the Watertown Mall it would probably have to run on Arsenal Street to Watertown Square.
- Watertown is currently benefiting from bus-priority lanes on Mount Auburn Street between Watertown and Harvard. Arsenal needs similar treatment.
Much less realistic, but ever so often discussed, is a Blue Line extension from Kenmore via the Mass Pike and North Beacon Street (or North Beacon all the way) to Watertown Square and then vaguely-handwavily to Waltham beyond. Abstractly, there's some real sense to this -- from Kenmore, this is indeed one of the clear transit cavities that would be accessible, and rapid-transitifying the Boston & Albany along the Pike is not going to help you much in Watertown, so there would still be a "niche." But... it would be astronomically expensive and challenging, and both Watertown and Waltham alike have such easier access from the north.
I've been noodling for months on whether a super pro-transit local government would somehow be able to run tracks the 1 mile from Sweetser Circle up Broadway to Ferry Street. As can be seen on Google Maps now, parking is being eliminated for (peak only?) bus lanes. San Francisco's MUNI uses street-running LRT with center-running bus lanes between general traffic lanes... but on streets that are a bit wider than Broadway. The best I've been able to come up with is... maaaaaaybe you could do it? It would obviously be easier if you could reduce the general traffic to one-way only, but there really isn't a good parallel street to pair one-way with. So there's a good chance you'd have to contend with street-running mixed traffic LRT, which is pretty much terrible.Everett:
- No easy way to bring rail to Everett. It gets to God-Mode pretty quick.
- Urban Ring Northern Route would give busses from Everett another linkage to more locations.
- Bus Lanes on Broadway are helping speed busses to Wellington/Sullivan
That all being said, I think it's worth noting that it's only (literally) that last mile would poses possibly-insurmountable challenges. LRT to Sweetser Circle would be a project, but hardly the most complicated one -- run along the commuter rail ROW down to Sullivan, hook over through the yards into the GLX expansion, and either run trains into the Central Subway or down the Grand Junction. (If you're stuck with mixed-traffic street-running, I'd say better to send down the Grand Junction so as to reduce the impact of the inevitable delays.)
I agree that rail to Everett doesn't have an easy way, but I'm a little more skeptical that you have to jump to God-Mode so quickly. ("You say to-may-to, I say to-mah-to.")