How might the new Metro loop work?

Last week, WMATA planners released a proposal for a new Metro loop downtown to help relieve capacity issues on the other lines. How might this new line operate? Details are scarce, but we can talk about some possibilities.

The proposal is to build a new Metro line that would loop through downtown DC. Starting at Rosslyn, the tracks would tunnel under the Potomac to Georgetown. They would then follow M Street and New Jersey Avenue to Union Station. The tracks would turn south along 2nd Street to cross Capitol Hill, and then parallel the Green Line under I Street. The loop would complete itself by joining to the existing Yellow Line bridge over the Potomac River.

Metro’s proposal indicates that the loop line will be fed by the current Blue and Yellow lines, which would enter at the Pentagon station, and a new “express” line in northern Arlington, which would enter near Rosslyn.

Why build a loop?

Metro’s team of planners looked at a variety of solutions to the core capacity issues, including new lines. The alternative they settled on was this loop. Why did they pick a loop?

Essentially, a loop solves most of Metro’s problems relatively cheaply. The primary issue facing Metro over the next few decades is core capacity, espcially in terms of train throughput. The Blue/Orange subway through downtown is at capacity, and no more trains can be added.

Untangling the Gordian knot at Rosslyn is the most complicated part of this. What is most clear is that Metro needs a new Potomac crossing near Rosslyn to increase capacity on the Blue, Orange, and Silver lines.

But Metro also needs more capacity around the southern side of downtown DC. Because the Green and Yellow Lines share the 7th Street subway, which is operating very close to capacity, each line can only be increased at the expense of the other line. In other words, WMATA can’t add Green trains without subtracting Yellow trains. So Metro also needs a new subway for the Yellow Line.

However, Metro’s studies found little need for a new subways outside of downtown based on the expected travel patterns in and density of those areas in 2040.

Essentially, Metro sees the need to build an east-west subway across the north side of downtown and a north-south subway across the east side of downtown. But they don’t see a need for the east-west subway to continue east or the north-south subway to continue north.

For that reason, Metro thinks it makes the most sense to just connect the east-west and north-south subways at Union Station and operate them as a loop.

Operating patterns

It’s not exactly clear how the new subway loop would work, and since this project is decades away from completion much could change. But there are a few likely ways it could operate.

For now, let’s only consider the Blue and Yellow Lines. We will discuss the proposed North Arlington Express line in a later post.

Operating an inner and outer loop is one of the obvious choices. In this scenario, one of the lines would run clockwise around the inner loop, while the other line runs counterclockwise around the outer loop.

So Yellow Line trains coming from Huntington would cross the Potomac north of the Pentagon, as they do today. Then they would continue east to Capitol Hill before turning north toward Union Station and going around the loop to return to Huntington via Arlington Cemetery. Blue Line trains would do the reverse.

One of the advantages of operating the Yellow and Blue lines as a loop through downtown is that the loop can actually carry more capacity. If the Blue and Yellow didn’t need to share tracks with the Orange and Green lines, each could run at a frequency of 13 trains per hour (TPH) in each direction. And that means that the outer loop would have 13 TPH, as would the inner loop. Since Metro’s track capacity is 26 TPH, there’s actually room to add two more lines to the loop.

One of the disadvantages, though, is that riders who have a short one-seat ride in the morning have a long one-seat ride in the afternoon. Someone who commutes from Franconia to Georgetown has a pretty direct trip in the morning. But in the afternoon, they either have to face a long ride on the Blue Line via Union Station and Potomac Park or take a two-seat ride by riding the Yellow Line to Pentagon and changing.

“Transforming” loop trains would resolve that problem, though it would be more complicated and difficult to show on the map.

In this scenario, a Blue Line train leaving Franconia would run as far as Pentagon and then continue toward Arlington Cemetery. At some point on its journey, the train would magically transform into a Yellow Line train bound for Huntington. Yellow Line trains would operate similarly, becoming Blue Line trains during their journeys.

This way, a person who commutes from Franconia to Georgetown would have a short, one-seat ride on the Blue Line in both directions. The same would be the case for Yellow Line riders.

For anyone waiting for trains at a station on the loop, trains on the outer loop would always be bound for Franconia, but would have come from Huntington. On the inner loop, trains would be bound for Huntington, but would have come from Franconia.

To avoid confusion, trains bound for the loop would just be signed with their color and a destination of “Downtown.” The change of color and destination on the loop wouldn’t matter for the passengers on board, since the train would continue around the loop. This is, incidentally, what Chicago L trains do as they arrive at the Loop: they change their headsigns from “Loop” to whichever destination they’re headed back to.

Alternatively, trains don’t have to “loop” all the way around the loop. Instead, the Blue and Yellow lines could just be interlined on the new tracks.

In this scenario, Blue Line trains would operate onto the new tracks for a certain distance. On the map above, I’ve shown trains going as far as 4th & Eye, but they could stop at any point along the line (Union Station, for example). Then trains turn back around and run over the same tracks back to Franconia. Yellow Line trains would operate similarly.

Anywhere the two lines overlap would max out the capacity of the new line, just as the other lines in the core are currently topped out at 26 TPH. The reason this is true for the interlined scenario but not for the loop scenarios is because in the loop scenarios, trains run around the loop once. In the interlined scenario trains run over the tracks twice, once inbound and once again outbound.

North Arlington Express trains

As noted above, the new loop would also carry trains from the North Arlington Express line. We haven’t discussed those trains yet, but we’re going to cover how they might work with the loop in another post soon.

Metro’s vision for the future is still decades away, so we have no idea what the final product will look like exactly. It might look like one of the operating patterns shown here, but then again, lots can change in 25 years. But Metro’s core is approaching capacity, and expansion is desperately needed. Metro’s new vision will set the stage for building the system’s next generation.