NS is still not well connected to the rest of the network, and that's got to be a factor too.
Even under RUR there are going to be some places where you convert "CR" right of way to LRT/HRT. GLX is a really good example (particularly if in GLX3 we go from Union to Porter, and OLX happens to allow capacity to be consolidated on the south side.
It also depends greatly on where people are going. GLX is a lot like all the rest of Green...a very high share of quick intra-neighborhood trips where it's co-absorbing the functionality of a very fast and high-capacity bus in addition to piping the CBD and transfers. There are many Brookline residents, for instance, who ride the C every day without ever leaving the Town of Brookline. You'll see some of that in Somerville, too...people doing Gilman-Ball and other pairings for business about town. You don't see as much of that with the makeup of the CR/Urban Rail corridors, simply because the ROW's are laid out in much more inner vs. outer/home stop vs. CBD fashion, tend to chop up the street grid a lot more this side vs. that because of the wholly 20th century grade separation retrofits on the first 10 miles unlike rapid transit's more organic mixed surface/subsurface portfolio, and the stop spacing is inherently wider. Even at :15 minute frequencies and a subway fare, there's not going to be a lot of people jumping between Uphams Corner and Talbot Ave. on quick trips like it's a drop-in substitute for bus mode. But the B is so chokingly sardine-packed on a normal Friday night because half of BU is doing campus-to-Allston short hops.
Factor in as well that even the most nimble RUR vehicle isn't going to platform half as fast as a trolley, and despite potential availability of tap transfers the fare portability isn't as set-it/forget-it as literally going behind the walled garden and staying there behind fare control for the entire multi-legged duration of a subway trip. You net modes that really aren't drop-in replacements for each other. Story of GLX's demand studies--going all the way back to 1945--is that there were clear and distinct X-factors favoring rapid transit. A lot of that owing to the fact that present-day routes 87 and 88 (both Top 35's in ridership) flanking this corridor were both former subway branches...then later cross-platform transfers behind Lechmere fare control along with the 89 and 90 streetcar/TT's originating behind Sullivan Orange Line fare control. Those stars don't align by accident.
It's similar to how BLX-Lynn and RUR-Salem aren't interchangeable--and in fact
both need to happen for full effect across the North Shore. Divergent destinations, and only BLX holds the trump card for mending the breakage of Lynn bus terminal holding back last-mile frequencies across the North Shore...caused by 49 years of equipment imbalance on the distended runs all 4xx routes make to Wonderland and through the Harbor tunnels. Every time someone lazily vomits out "well just build Wonderland CR across the parking moonscape and be done with it" they totally miss the point (at least until the latest ridership projections math out just as shit as the last 5 times it was studied) that hardly anyone's getting from home to there without a car so long as all the 4xx's are sentenced to another generation of running half-cocked from broken cycling.
You will usually see some big whopper of a tell like that in the vital-signs demand data when a Preferred Alt. comes firmly on the side of rapid transit. I mean, nevermind that the Grand Junction physically can't run an RUR dinky frequent enough to serve Kendall a drop in the bucket...CT2 intersecting 1/CT1 + Red and the 66 badly needing an alt-pipe to keep the Longwooders and Brookliners from choking on each other's overcrowding perfectly traces out the Urban Ring NW + Harvard Branch for a blind man. Eight routes clipping Sweetser Circle and another six in/around Box District says something about the Ring NE quadrant between Sullivan and Logan. So does 4 & 7 overcrowding point to some overly pregnant need for a Back Bay-Seaport thru connector. So does 8 routes duplicating each other on Lower Washington to Rozzie. Or the alternating quadruple-ups on Blue Hill Ave. pointing to a 28X corridor that's real feature-complete BRT, no half-assing. Hell...even with frequencies currently really lame after repeat service cuts you can see what they were thinking with OL-Reading with the bundle of 131/136/137/106 tracing out the entirety of that corridor.
I think in contrast you can see where some of the Urban Rail lines are collections of nodes rather than continuous corridors. Fitchburg Line to Waltham/128 has good demand because pretty much each individual stop is anchoring the end of the line for some major bus...but Waltham vs. Waverley vs. Belmont vs. Porter really aren't oriented to/from each other. That's much more home stop vs. Red Line/CBD travel pattern...though Waltham being a big bus hub means some of those long-jump pairs are going to be pretty hot on demand.
Fairmount...much denser overall but sort of same vibe. The mega contiguous corridors are the 28X and Ashmont Branch/Ashmont Hub, and the Fairmount Line stations are mainly traversed by east-west routes. A lot of east-west routes, which is why the ridership studied out good...but B Line on a Friday night this is not going to be because the intra-neighborhood orientation really isn't along the north-south Fairmount corridor.
Likewise, when I say that BLX and RUR are both needed for the North Shore you can see from where they trace south of Lynn why one to the exclusion of the other isn't going to cut it. There's no bus connectivity whatsoever between Lynn Terminal routes and the Maverick Terminal routes that source Chelsea...mobility between the two cities is all tied to Blue as the optimal intermediary. But if you're on the upper North Shore you are eyeing the long-jump to North Station more lustily than the very long commute on the full length of Blue..all provided that you don't need to drive to that albatross of a downtown garage because BLX fixed the Lynn Terminal bus rotations and afforded enough equipment for Salem to establish its own heft as last-mile feeder hub. There are telltales for things breaking that way, too.
There's no 'universal' mode. Never was. Thinking there ever was a one-size-fits-all is how incoherent executions like the Silver Line were allowed to get as incoherent as they did and spit right in Captain Obvious' face. Rapid transit and RUR are distinctly different tools for the job. As we see, mixing and matching the tools simultaneously is what builds the mighty house. One-size-fits-all you've just got one hammer seeing everything as a nail, whether the actual task at hand is calling for a Phillips-head screwdriver or circ saw. Not a mentality that lends itself to well-executed results.
