Originally Posted by bkong
If I remember correctly, the present tunnel under the Potomac is only one track and that's already a choke point with all the Orange and Blue trains during rush hour. What I read wasn't clear about how Metrorail might re-route Blue trains to the Yellow Line bridge across the Potomac. A majority of Virginian commuters who currently board the Blue Line in the mornings work in Northwest DC so they'd need to transfer at L'Enfant if their Blue trains take the Yellow Line bridge. Also, there'd need to be maintained a reasonable level of service to Arlington Cemetery.
I'm pretty sure the Potomac tunnel is two-track wide (one in each direction - perhaps that's what you meant?). Because the line is overcrowded, Metro is sending more and more trains at shorter intervals through the tunnel which results in trains backing up under the Potomac between Rosslyn and Foggy Bottom. The "inching" through the tunnel happens almost everyday and it almost never happens on the other lines.
If they send the Blue Line up the Yellow Line bridge, Blue Line VA commuters will just adapt to the regular Yellow Line VA commuters that transfer at L'Enfant for Orange/Blue and Chinatown for Red. Perhaps one out of every 3-5 trains during rush hour will continue to use the regular Blue Line route, thereby significantly enhancing Orange Line capacity through the Potomac tunnel. The only drawback is a confusing Metro map (but other cities - like London - already deploy the "this train goes this way during rush hour but this other way during all other times" method).
Originally Posted by bkong
My guess is also that any new tunnel would be at Georgetown. DC's a different city now and Georgetowners are really pining for a Metro station to the point that many of the merchants said they'd help pay for it, according to the Post a while back.
The $12bn plan was to push a tunnel from Rosslyn to Georgetown and all the way down L or M St to Union Station via the Convention Center. That would make sense since there is no truly cross-town line at the moment. And judging by the crowds that get off at Dupont/Farragut every morning, an L/M St line would be heavily used.