Welcome to European Tribune. It's gone a bit quiet around here these days, but it's still going.
How tight?

I don't know ... I have seen claims they are too tight to be workable. Other than vague happy talk, all I can find is the fuzzy diagram from a 2003 document that was part of the Environmental Impact Report process.

When I say in the post that I don't know if its physically possible ... that's part of it. Swinging the central platform tracks so they come in #2 and #3 from the outside, and the tail track so they go out #1 and #2 from the outside, buys some straightening. Probably not enough, though.

When I was sketching out an operations table, I included one empty slot for four full slots on the access track for delays.

On the CAHSR blog, Brandon in San Diego pointed out that Prop 1A requires 5 minute headways on the HSR network, so on the operation model above, with alternation between HSR and Caltrain built in, an argument can be made that 2 1/2 minute headways would be required by the funding legislation. Tighter headways does not increase platform capacity, so if they can arrive at 2 1/2 minute headways, that could be one open slot for each pair of inbound services.

Its parallel arrival HSR / departure Caltrain and parallel arrival Caltrain / departure HSR that is the point of having an egress track on either side of the access track ... eliminating that crossover between access and egress tracks at grade (or at least, putting it off to the mouth of the tunnel, when its a "half-dive" ... cheaper because the tracks have to dive into the tunnel in any event) ... is what that accomplishes.

As for bidirectional operations on the Caltrain egress track, if the Caltrain egress track is taken down first, to dive under the common access track to the tunnel, it can be a central branch, switched to both sides of the two way track out of the current terminal at 4th, which is about 2km away.

I've been accused of being a Marxist, yet while Harpo's my favourite, it's Groucho I'm always quoting. Odd, that.

by BruceMcF (agila61 at netscape dot net) on Wed Mar 25th, 2009 at 11:42:03 AM EST
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