Photo by the author.

Last week, we talked about plans to give Metro the capacity it needs to get through 2025. What about beyond? The primary issue after 2025 will be cross-Potomac capacity.

Metro will likely choose to build, hopefully by 2025, either a new terminal for the Blue Line at Rosslyn that doesn’t share tracks with the Orange or Silver Lines, or a “wye” so trains from Vienna and Tysons can turn toward the Pentagon.

However, neither solution increases the number of trains that can cross the Potomac River. Metro will need to start planning for the next phase very soon, since it takes so long to plan and build transit. Also, Metro’s plan beyond 2025 could influence which of the options (terminal or wye) it chooses for 2025. Let’s look at how Metro might expand its core capacity starting with each of the 2 primary Rosslyn alternatives.

Post-2025 solutions that go with a Rosslyn wye

Without construction in the core, a Rosslyn wye alone can’t add any cross-Potomac capacity. Metro could build a second wye at Pentagon, for trains from Vienna and Tysons to cross the 14th Street bridge. However, the 7th Street tunnel, which carries the Green and Yellow Lines through downtown, can’t take any more trains.

Wyes at Pentagon and L’Enfant: If Metro builds a total of 3 wyes, at Rosslyn, Pentagon, and L’Enfant Plaza, it could fit a few more trains across the river. Trains from Arlington Cemetery could cross the 14th Street Bridge, then continue east onto the Green Line toward Branch Avenue.

This scenario would let Metro fill unused capacity in the system without building any new trunk lines. However, much of this new capacity would go on the Green Line south of Waterfront station. Customers wanting to get to the downtown area would have to transfer.

It would, however, significantly add service to the growing Waterfront and Capitol Riverfront areas. DC has zoned much of these areas for downtown densities, but instead of the 5 lines that serve downtown or 3 in the Golden Triangle, this area has just one. This option would beef up service there, though many of the people riding there would want to come from downtown, and this doesn’t boost that connection.

To absolutely max out capacity, Metro would need to run a line, like the lime-colored line on the map above, from Franconia to Branch Avenue.  It’s not completely necessary, but it would allow more full use of the capacity.

Unfortunately, this would increase the amount of interlining in the system, because trains would be running across multiple lines. The complex scheduling it would take to run this sort of service pattern might actually lower the total number of trains Metro can run through a tunnel.

This scenario would increase trans-Potomac capacity from 40 to 52 trains per hour, or just to 46 TPH without the Franconia-Branch Avenue Line.

Pentagon wye and separate Yellow Line: Metro has talked about building a separate tunnel for the Yellow Line. It’s not clear where it would go yet. It could run north along 9th or 10th Street, or it could run east toward Capitol Hill before turning north.

This subway would separate the Yellow and Green Lines. That would allow Metro to run additional Green Line service between Greenbelt and Branch Avenue, including more service between downtown and the Waterfront/Riverfront areas.

It would also allow more service across the Potomac by decoupling the Green and Yellow Lines. However, since the Yellow Line shares tracks with the Blue south of Pentagon, this new subway would not be used to its full capacity.

Metro could get some additional capacity by routing some trains from Tysons via Arlington Cemetery over the 14th Street Bridge and into the new subway, if it built a wye at Pentagon. This would increase service across the Potomac to 52 TPH. Without the line running from Tysons through Arlington Cemetery, that would drop to 46 TPH.

Splitting Yellow Line service: Another option Metro studied for the Yellow Line is a new tunnel through the Capitol Riverfront, past the Capitol, and north to Union Station. This would increase service at Union Station, a major bottleneck, and give riders two ways to get downtown from the Waterfront/Riverfront area.

However, a lot of the riders in Alexandria and southern Arlington don’t want their train to go so far east. They want to get to the 7th Street corridor. Therefore, Metro studied the idea of splitting the Yellow Line, with some trains taking their current path through L’Enfant Plaza and Archives while others would go to Union Station.

This operating plan makes a lot of sense with the Rosslyn wye, because those trains can fill the “gap” left on the 14th Street Bridge by Blue Line trains running north toward Rosslyn. With a new path for some Yellow trains, there would be room to add more Green Line service.

This approach would allow 52 trains per hour to cross the Potomac.

Tomorrow, we’ll look at another set of long-term solutions, which Metro might pursue if it builds a new Blue Line terminal and then can send the Blue Line across the river toward Georgetown.

Matt Johnson has lived in the Washington area since 2007. He has a Master’s in Planning from the University of Maryland and a BS in Public Policy from Georgia Tech. He lives in Dupont Circle. He’s a member of the American Institute of Certified Planners, and is an employee of the Montgomery County Department of Transportation. His views are his own and do not represent those of his employer.