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NikolasM -
Care to elaborate? I was figuring maybe $1-1.5B to reconstruct Rosslyn station for another approach and put in a Pentagon bypass(or simply a pocket turnaround track), plus however much is necessary for a new Yellow tunnel across the city (what, $5B or so to get to Silver Spring and another $2.5B to White Oak running in a semi-cut-and-cover median?).

I'm just pointing out that if you're considering a pricy deep-tunnel separated yellow line as the basis for Metro expansion (rather than a separate blue line), doing some a little trackwork to max out the two river-crossings at 10 trains per 12 minutes makes sense. Right now there are seven trains crossing the Potomac every twelve minutes. As the original post has it, you've still got only eight or nine trains crossing the Potomac every twelve minutes (depending on whether you split the Blue).

A Separated Blue could handle 10 trains / 12 min as well.

I'm assuming, of course, that the track switches and train control program can handle (or be made to handle) this safely - a big assumption.

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stevek_fairfax -
I examined that possibility about six months ago as one of the only viable ways to handle the Silver Line without a Separated Blue, and indeed found several pocket tracks and three-way junctions have been identified by WMATA to study (damned if I can find the PDF for it though). My idea involved the Silver using the rest of the Green Line track - focusing on the minimal-investment option necessary to handle a large Silver Line ridership without crippling the rest of the system.

Potential problems included:
If the switching/train control really isn't up to it, this greatly increases the number of line merges WMATA has to do;
Difficult near-surface tunneling in watery ground at Pentagon & Washington Channel;
The fact that if you want to make these 3-way junctions cheaply, they have to completely bypass the station;
The need for a new transfer station at East Potomac Park(not much of a problem);
The end of the "only 1 transfer necessary to any destination" policy that Metrorail currently uses (not much of a problem)

by Squalish on Jun 29, 2009 11:12 pm • link • report