A cheaper route to Metro core capacity, part 2: Virginia service patterns
One of the possibilities from Metro's core capacity study involves a short 9th Street tunnel from L'Enfant Plaza to Mount Vernon Square. That could be a cheaper way to add Metro capacity across the Potomac, the system's current bottleneck. It wouldn't add service to much-needed areas like the McMillan/AFRH area of DC, (though allowing that possibility in the future), but would address the impending overload of trains from Virginia once the Silver Line opens.
If we could run more trains over the 14th Street bridge, where would they go in Virginia? I can see two possibilities: convert the Arlington Cemetery segment to a shuttle train, or add connections to route the Silver Line over that segment as well as the Blue Line.
The shuttle train option
The Blue Line could simply run over the 14th Street Bridge with the Yellow Line to the new section. Silver and Orange share some tracks, Blue and Yellow others, with no other merges. To replace Blue at Arlington Cemetery, create a shuttle train
To avoid having the shuttle merge and unmerge with Orange/Silver at Rosslyn and Blue/Yellow at Pentagon, Metro would need to add a new platform at each station. Probably this could work with just a single platform inside the station, with a switch as close as possible. Trains would come in, unload and reload, then reverse out to make room for another train. Such an arragement would limit the capacity on the line, of course. Ideally, the platform(s) would go right across the platform from one of the two existing tracks in each station, minimizing the walk necessary to tranfer.
Pros: This requires fewer merges than in the current arrangement. Also, all merges happen outside of the highest ridership core areas, minimizing delays.
Cons: Commuting from Alexandria to "Orangeton," or southern Fairfax to Tysons, becomes more difficult, requiring either two transfers or a trip through DC.
The new connections option
Squalish got this one. In Metro's earlier core capacity study, they suggested adding some track connections for service flexibility. Those included a connection from Court House to the Arlington Cemetery tracks, and another one from the other end of those tracks to the 14th Street Bridge. If we built both of those, then the Silver Line could use the track in the opposite direction from the Blue Line, to get from Court House to L'Enfant Plaza.
Pros: There are lots of services going to lots of places. Riders along Rosslyn-Ballston or King St-Pentagon can choose either bridge. And except going to and from Yellow Line stations south of King Street, riders can go between any two Virginia stations entirely within Virginia with at most a single transfer.
Cons: Lines are merging and unmerging a lot, which creates operational challenges. Silver and Blue each have to share tracks with three other lines for part of their routes. If all lines are running at capacity, then at Pentagon (for example), a Silver Line train needs to reach the wye just as a Blue Line train reaches it from the other direction, or else one of the trains will have to wait, delaying all later trains.
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