When WMATA released its Momentum plan last week, it reopened the conversation about dealing with core capacity. By 2025, the plan seeks to address one of the biggest chokepoints in the system: Rosslyn.

View peak service levels: Pre-Rush Plus Rush Plus (today) with Silver Line Potential 2025 solutions: Rosslyn wye Blue Line terminal

Metro has to juggle service at Rosslyn, where the Orange and Blue lines merge entering the District from the west, because it faces a structural limit to the number of trains per hour (TPH) that any given section of track is capable of handling. That limit is 26 TPH, or about one train every 2.3 minutes.

The reason for Rush Plus last year was to deal with capacity issues at Rosslyn without undertaking any capital projects. Essentially, Rush Plus was a stopgap measure to get a little more capacity out of Metro. But Rush Plus hurt many Blue Line riders, and without major changes at Rosslyn, even Rush Plus won’t be enough.

As ridership increases, Metro has to either find creative ways to move more people throughout the system, without crossing 26 TPH on any segment of track, or expand capacity.

Before Rush Plus

Before Rush Plus started, during peak periods, exactly 26 TPH (the maximum) passed through Rosslyn. Those 26 TPH consisted of 10 Blue and 16 Orange trains.

At Pentagon, those same 10 Blue trains were passing through in addition to 10 Yellow trains. In total, 36 TPH were entering the District from Virginia during the morning rush via Rosslyn and Pentagon combined.

But this left some capacity unused. The Yellow Line bridge was carrying only about 40% of the trains it could carry. As you can see in the diagram below, there was also extra capacity in the Yellow and Green Line tunnel.

This map shows the frequency of service for every line during the peak period. Every track segment is the same width, corresponding to 26 TPH. If 26 TPH pass through at peak, it’s “full” with colored lines; if there are fewer trains, there is black space.

After Rush Plus

Once Rush Plus took effect, those numbers increased slightly. At Rosslyn, the numbers are now 19 Orange and 7 Blue, which is still 26 trains per hour. But at Pentagon, in addition to the 7 Blue trains running via Rosslyn, there are now 13 Yellow trains. That means a total of 39 TPH are entering the District from Virginia during the morning rush.

Since the number of trains is the same at Rosslyn as it was before, Metro could have kept the number of Blue and Orange trains the same, and just added new Yellow trains. But adding trains from Virginia was only part of the equation. Metro was also attempting to address the severe crowding on the Orange Line and paving the way for the Silver Line. As a result, Metro traded some Blue Line trains for Orange ones.

When the Silver Line begins operating, some of the Orange trains will have to become Silver trains, in order to keep the total number passing through Rosslyn at 26 TPH.

Cross-Potomac capacity will be maxed out

One of the biggest constraints with Metro is getting trains between Virginia and DC. There are 2 Metro crossings of the Potomac: a tunnel for the Blue/Orange lines and a bridge for the Yellow Line.

Each of these crossings has a capacity of 26 trains per hour, for a total of 52. However, because the Yellow shares with the Green Line, the Yellow Line bridge can only carry 26 TPH minus however many trains per hour are running on the Green Line (at present, 12 TPH).

This means that there are 14 slots available for the Yellow Line to cross the Potomac. Right now, the Yellow Line is taking 13 of those slots during rush hour: 10 for trains running between Huntington and Mount Vernon Square and 3 for trains running between Franconia and Greenbelt.

When the Silver Line opens, Metro’s plan will be to redirect one more Franconia train to Greenbelt. This will mean cross-Potomac capacity will be maxed out at 26 TPH through the Rosslyn tunnel and 14 TPH over the Yellow Line bridge, for a total of 40 TPH between Virginia and DC.

Through 2025, those 40 trains are probably enough. But there’s another problem: now there aren’t enough trains running between Pentagon and Rosslyn. Riders from south Arlington, Alexandria, and southern Fairfax have long waits to get a train to Rosslyn, the Orange and Silver corridors, or to reach stations like Foggy Bottom without a transfer.

How can Metro fix Rosslyn?

Metro wants to address the Rosslyn chokepoint soon. It’s one of 7 capital items in the “Metro 2025 recommendations of the Momentum plan.

Planners haven’t decided on a specific solution yet, but are studying 2 options. One would build a “wye” at Rosslyn, a track connection so that trains from Tysons or Vienna could turn south and head for Arlington Cemetery, and vice versa. The other option is to build a new station at Rosslyn with separate platforms for the Blue Line.

The wye at Rosslyn would also let Metro add rail service between the Blue-Yellow corridor and the Orange-Silver corridor. Some trains going through Arlington Cemetery would go toward downtown, as the Blue Line does today, while some trains would turn west and run through Ballston and Tysons.

This approach would likely mean a good deal of disruption for Orange and Silver line riders, since Metro would need to build new underground connections to the line between Rosslyn and Court House. It would also permanently limit the number of trains between Court House and downtown DC, because some of those slots would go to the Blue Line and the new north-south all-Virginia line.

But this would also give riders a one-seat ride from Tysons to Pentagon and points south. It would also build a little more redundancy into the system, allowing trains to divert to the south in the case of track work, a disabled train, or other disruptions.

The real issue, though, is that this project only allows for a small increase in the number of trains, and does not increase the number of trains between Virginia and the District. It also would mean that some trains wouldn’t stop at Rosslyn, forcing those actually heading there to wait for a later train or transfer.

Under the wye scenario, the service pattern would probably look something like this: 14 Yellow trains crossing the 14th Street Bridge (4 of them from Franconia) each hour during the peak; 5 TPH on the Blue Line between Franconia and Largo via Rosslyn; 10 TPH on the Orange Line from Vienna into DC; and 11 TPH on the Silver Line from Tysons. This still adds up to 40 TPH between Virginia and DC. In addition, the wye would enable 5 completely new TPH between Pentagon and Court House.

A new Blue Line terminal at Rosslyn involves building new, separate tracks at Rosslyn, possibly in a new station next to the old one, that would become the new end of the Blue Line. Blue Line trains could stop and turn around without interfering with the Orange and Silver trains going to Foggy Bottom, and Blue Line riders could transfer to either direction of those trains at Rosslyn.

A new Blue Line station at Rosslyn would allow even more trains between Pentagon and north Arlington — 12 TPH, compared to 10 for the wye.

It would require passengers to transfer to the Orange or Silver lines to get to downtown or west toward Ballston and Tysons. But it would also form the first step toward a separated Blue Line through downtown. If and when Metro is able to build that, it could simply extend the new tunnel under the Potomac and beyond. If WMATA thinks a separate Blue Line will be the long-term approach, this solution might be more attractive.

Another advantage to a Blue station at Rosslyn is that it reduces the amount of interlining in the system. That should have a positive effect on reliability, because a delay on the Blue Line won’t affect the Orange or Silver lines, or vice versa.

Note that we don’t know which trains will go to Franconia or Huntington under this or any other future plan, or how many and which trains will go to New Carrollton vs. Largo vs. turn at Stadium-Armory. Since all trains to those destinations come from the same track, it’s not an operational issue, just a question of balancing simplicity with convenience and being fair to riders on each branch.

Transferring can be painless

If Metro does go with the new Blue Line station at Rosslyn, they can help make transferring painless by getting the design of the new station right.

Cross-platform transfers are more commonplace in some cities. Montreal, for example, has 2 in their Metro system, Lionel-Groulx and Snowdon. New York has many, especially between local and express services on each line.

A 4-track stacked Rosslyn station would allow passengers from an inbound Blue Line train to simply walk across the platform to board an inbound Orange or Silver train. To transfer from an inbound to an outbound, the passenger would just have to go downstairs or upstairs.

The station could look something like this:

A Rosslyn design with cross-platform transfers. Graphic by the author.

It would be more expensive, but in terms of simplifying riders’ ability to transfer, it could be worth it to build a new Rosslyn station with 4 tracks.

On the other hand, such a station would cost more, and would disrupt all service through Rosslyn during construction, just as the wye would, while a separate station could avoid interfering with Orange and Silver service.

But any new station will be there a long time, and riders may regret making it more difficult to transfer. After all, many riders still suffer from the lack of a direct connection between Farragut North and Farragut West (a tunnel that Metro proposes to complete as part of their 2025 plan).