I'm also assuming this would kill any Grand Junction GL proposals. Nice concept though.
While it's technically possible to have LRT(sort of) and RR sharing tracks with time-separation like they do in New Jersey, it'd likely be completely infeasible here. For one thing, do Green and the CR even use the same wheel profiles? River Line in NJ doesn't have to run into an extant LRT system like Green-on-GJ would. Moreover, even getting a time-separation agreement would probably require CSX's approval given that they have freight trackage rights over the GJ (even if they aren't using them at the moment) and they might balk at that (at least without a MassDOT quid-pro-quo). All that's basically moot anyway, given that the nature of temporal separation means that one of the modes (presumably RR) would be consigned to operating only when demand was low (probably overnight), so it wouldn't even be worth running service. Taking GJ off the RR mode is feasible, sharing it is not, so, yes, CR via GJ would kill GL over GJ, and vice-versa.
The potential for through-running Amtrak! In a speculative future with real American HSR, the only realistic HSR routes will be Boston - NYC/DC and Boston - Chicago. North of Boston is too thinly settled to have HSR-worthy demand, so a South-to-West through-running pattern could make sense.
What does this mean? Even assuming some
very generous average speeds for this Crazy Transit Pitches unlimited-money HSR service, BOS-CHI would still take something like 5-7 hours (realistically considerably more), whereas a flight takes 2-3; even if you factor in getting to Logan, security, baggage, and the atrociously-long trip in from O'Hare, you're probably talking a total travel time not far off from the absolute-maximum possible for HSR...except that HSR would cost a bazillion dollars just to bring into existence whereas the aviation infrastructure
already exists. That's not Crazy Transit Pitches, it's the God Mode thread. Even leaving that to one side, Amtrak would almost certainly prefer to run any Chicago HSR service to its main terminal at South Station; there's no need for the GJ for a service that, if it ever existed, would inevitably have Boston as its terminal.
If Amtrak runs trains through Boston, it could also relocate its yard to somewhere less expensive than downtown.
Amtrak owns its Southampton Street yard, it'd take the mother of all offers to pry them loose from there (does make me wonder if Amtrak is even legally subject to state-level eminent domain laws). They're never going to have any meaningful interest in running the
Acelas through an NSRL, so it's not very likely that they'd be interested in moving their operations to a less-convenient location, at least absent significant incentives from the state to do so. That
also has no bearing on the GJ, because it'd only ever come up in an NSRL world.
GL is a lot more do-able than CR, both technically and politically
As a full-scale service, I agree with you. As an "add-on", I'm not so sure. CR-over-GJ wouldn't require
that much work (in comparison to GL-over-GJ). I could envision Worcester-to-North Station via GJ making a comeback (and getting done this time) as a Beacon Hill political gift to Worcester should the next administration have a neo-Tim Murray wanting something like that. Political baubles (cough*SCR*cough) are, of course, occasionally immune to questions about sensibility, utility, and cost-effectiveness. As something more than a political trinket, though, CR-over-GJ's hard-fail on the cost-benefit analysis compared to GL-over-GJ would absolutely hamstring it.
