I agree with your overall philosophy here
@TheRatmeister, so I don't want to come across as combative, but I do want to drill in to some of these points more specifically:
If you're linking into the GL system, you need low-floor vehicles, which are not and can never be 'heavy metro' due to their more limited capacity stemming from the fact you need to put all the wheels and guts under the floor somewhere. This is why LA uses high-floor LRVs, not low-floor trams.
There are more characteristics of heavy metro than just outright capacity. But either way, a grade-separated, limited-stop service is going to be higher capacity overall than an intersection-traversing surface-stop-spacing service (assuming same vehicles) due to the shorter travel times.
I've been trying to find data to support your claim about high-floor vs low-floor capacity, and I must be looking in the wrong places -- can you help me out?
And as for low-floor vs high floor itself -- MUNI Metro uses converting cars that can serve both low and high platforms, and LA's A Line itself demonstrates that street-running segments can, eventually, be run with high-floor trains.
It also ignores the fact that you can have surface running light rail that is quite fast. Again in LA, parts of the A Line have an average speed of 28 miles per hour. Granted, the stop spacing is very high, more like 2 miles than the 2000ft that makes sense on Washington St, but I think it's definitely possible to at least get closer to the ~15mph average speed you need to match the El's travel time than you think it is.
Does the A Line do 28 mph in its street-running segments, or along its dedicated ROW? Average speed of 28 miles per hour means that the typical
running speed has to be something extremely high like 40mph, which is never going to be appropriate for a city street like Washington. (Unless you actually mean "28 mph when running"?)
The long stop spacing is part of my point: if you are making stops every 2,000 feet, the service is gonna be slower. And reducing the number of stops reduces the benefit to South End residents, who will need to be onboard for this infrastructure to be built. Running via I-93 allows you to reduce the number of stops.
Parts of the C branch, with no signal priority and a bajillion intersections, already have average speeds around 10mph. (On Washington St this would look like 11 minute travel times vs ~7 to match the El.)
Maybe I used different data in my analysis, but as far as I can tell, the Central Subway itself between Kenmore and Park barely reaches 10 mph. Can you provide more detail on where and how the C reaches 10 mph?
At the end of the day, to match the El's travel times with a surface branch, you would need to run something like the Orange Line at street-level. I don't see that happening, unless you install quad-gates and always give the light rail the priority when crossing through intersections. (And I don't think that's feasible.)
And arguably an even bigger issue, this completely ignores the fact that if we're concerned about getting people from Dorchester and Mattapan into downtown as quickly as possible, the Fairmount Line is right there, right now. With today's diesel trains it can still get riders from Mattapan to South Station much faster than any bus+light rail combo ever could. (~22 mins vs a fairly optimistic 35 mins)
Yeah, this is a reasonable question and a larger debate that has been hashed out elsewhere on the board. But if Fairmount is sufficient, then significant upgrades to Washington shouldn't be necessary at all?
EDIT: I should be clear: if you can convince me that a surface LRT line on Washington St would achieve what I’m describing, I’ll be thrilled. I would love to be proven wrong. I’m just articulating the ways I’m not yet convinced.