Yeah, it's been a while since I was reading about this, and I was trying to piece all this together from really incomplete snippets of data, but:
The curve, though, you're right, that could be a problem. I have no idea the minimum allowed curve on a 2-3% incline. I don't know what that turning radius is, there. That might limit the speed there, or might be one of the many things that makes this plan fail.
As for the picture you posted, yes, some of the portals don't have that ventilation space immediately above, because the highway is too shallow to allow anything above it at that point. But the portals aren't usable for trains anyways. From Kneeland to South Station and from the Zakim to Haymarket, the new highways are 5% grade. That number is pretty clearly documented.
Yes, it's close, but I am pretty sure it never goes above 3%. That's piecing together lots of partial diagrams, construction photos, and planning documents mentioning depths at certain points, so I have no one document to point you to. It seemed possible it never goes above 2.5%. But, I could never pin that down absolutely certainly, so I'd love if someone just knows that answer. And has a source.One issue I see past haymarket is the pinch between state st and the blue line tunnel roof. I’m not sure if the grade exceeds 3% but it’s pretty darn close. There’s also the vertical curve right at the pinch which could cause issues for trains.
The curve, though, you're right, that could be a problem. I have no idea the minimum allowed curve on a 2-3% incline. I don't know what that turning radius is, there. That might limit the speed there, or might be one of the many things that makes this plan fail.
Yeah, I should have been clearer. That picture is from I-90, not I-93, but I think all the new Big Dig tunnels were built like that. That was just the clearest picture of what's above the panels. Again, that was hard to confirm for the whole length the tunnels, so if anyone here has a secret stash of CA/T engineering documents (probably) I'd love to look!I believe the photo in the linked post is of the Ted at one of the portals, which definitely has a large height clearance for trucks.
The CA/T, at least at the portals, has nothing between the girders and tunnel ceiling, as evidenced by past strikes by rigs:
As for the picture you posted, yes, some of the portals don't have that ventilation space immediately above, because the highway is too shallow to allow anything above it at that point. But the portals aren't usable for trains anyways. From Kneeland to South Station and from the Zakim to Haymarket, the new highways are 5% grade. That number is pretty clearly documented.
That makes sense. I guess I was trying to get at a couple of other things, though. 1) When they built the bus station and tower, did they do any kind of provisioning to maybe make it easier to one day build tracks under South Station? and 2) Whether or not they did that, how expensive would it be to build those tracks? Doing that now isn't impossible, but I'm sure it's very expensive. How would that very expensive station compare to, for example, mining a new station 10-15 stories under Fort Point Channel?Well the overbuild does span the entire width of the platforms, so I imagine the deep foundations extend the entire width as well. There’s also a major difference between building the stations before building the foundations and going back and beefing them up as part of the underground train shed later.