Dave at Imagine DC (and also GGW) put up a nice concept of a separated Yellow line through the core of DC.  Separating the Blue line has been the most popular suggestion, and was originally among WMATA’s official plans, but the idea of separating the Yellow line is relatively new.   Still, amongst extensive discussion in the comments from previous fantasy maps, the idea has come up before – my name is somewhere in those comment threads.

Given the focus that Monday’s accident has put on redundancies in the transit system, it’s fitting to consider the idea.  However, it’s important not to lose sight of the reasons for such plans and expansions in the first place.  With that in mind, I’d propose a few key principles to consider for any metro expansion plan:

• Separation of the current interlined portions of track.   The proposals to separate the Blue and Yellow lines certainly do this, and for good reason.  The ‘tail’ sections of each line are limited by the capacity of the shared track at the core.  Furthermore, the complexities of switching so many trains on and off the same line only adds to potential delays.  Separating these lines would offer wide ranging benefits to other lines in terms of increased service frequencies.
• Plan the entire system now.  By ‘now’ I don’t mean today, but if plans are drawn up to implement this kind of expansion, it is vitally important that the lines are planned together.  The fact that all 100+ miles of Metro were planned as a coherent system is what makes it such a useful system today, rather than a hodgepodge of individual lines.  If you look at the poor connections between Baltimore’s light rail and subway, you’ll see precisely what you wish to avoid.  That means planning to separate the Blue & Orange lines, the Yellow & Green lines, and the VA portions of Yellow & Blue at roughly the same time.  Doing so, like the original system, will allow for transfers to be built in and will make for a much better overall system.  Begin with the end in mind.
• Learn from Metro’s past. Metro’s hybrid nature as both an urban subway and a suburban commuter rail system makes for some interesting compromises in terms of system design.  Given that the newer portions would entail track mostly in urban areas, it’s important to apply the lessons of Downtown DC, the Rosslyn-Ballston corridor, and others.  This is about urban transit, not park and ride stations.
• Coordinate plans with other modes. Metro expansion should focus on the core because that’s where it’s most useful and can justify the cost.  Ideas like extending the Orange line to Centreville, or the Green line to BWI miss the opportunity to have a newly beefed up regional rail system operating in place of MARC and VRE trains.  Ideally, such Metro expansion plans would be coupled with a transformation of the commuter rail services into a more S-Bahn like system.  In the other (more local) direction, coordination with streetcar planning is also vital.

Speaking in terms of broad corridors, Dave’s plan for the Yellow line is spot on.  I think he’s got too many stations for a heavy rail line, but the general corridor is correct – the line would use the same bridge over the Potomac, then go underground and follow the Maryland Ave right of way, linking to L’Enfant Plaza with a new platform for the station complex.  The line would cross the Mall, then travel north under North Capitol, including a station at H street – which would also be part of the station with the new Blue line – also connecting to the current Union Station stop (hence the importance of planning these lines at the same time).  The line should go up to the Washington Hospital Center, serving that major employment center, then sliding to the west somehow to turn northward again under Georgia Ave.  After reaching Silver Spring, the line can either end there, or continue north along US 29, as proposed by Sand Box John.

The Blue line has been discussed many times – I think the best alignment would be across town under M street, angling southward under New Jersey Avenue, and then continuing east under H street – ideally with transfers to the Red line at both Union Station and under Connecticut Ave, as well as a Green line transfer at the Convention Center and a transfer to the new Yellow line near the Union Station complex.

Separating the Blue and Yellow lines in Virginia is probably the easiest route to conceptualize – simply shooting the line outward under Columbia Pike is the most obvious choice, making the current line to Alexandria one ‘color’ with multiple spurs – one to Huntington and one to Franconia-Springfield.