Photo by dbking on Flickr.

Rosslyn is a major bottleneck in the Metro system. Because the Orange and Blue Lines intersect at Rosslyn, the station can be a source of backups, especially if there are any problems.

Metro’s switches can handle a train every 135 seconds — 26 trains per hour — and that is the current throughput at Rosslyn. Ridership has been growing on the Orange Line, and some have called for WMATA to make room for more Orange trains.

Currently, the 26 spaces are allotted unevenly. The Blue Line has 10 slots while the busier Orange Line has 16 slots per hour. The Orange Line also has some 8-car trains, which give it a little more capacity. The Blue Line only operates 6-car trains.

Because of ongoing power upgrades and a lack of railcars, it is not currently possible to make all Orange Line trains 8 cars long. It might be possible to slightly increase the current number of 8-car trains, at best.

That means the only viable way to add capacity to the Orange Line is by adding trains at the expense of the Blue Line. And two years ago, Metro started thinking about doing just that.

Current (left) and proposed service (right).

Shows a given 12 minute period, but does not show the full TPH count.

The “Blue Line Realignment” — sometimes called the “Brown Line” — would redirect just under half of peak period Blue Line trains to run from Franconia to Greenbelt, crossing the Potomac on the Yellow Line bridge. Metro says they will probably sign the trains as Yellow since they run almost entirely on the Yellow Line route).

This proposal would allow for the addition of 4 more Orange Line trains per hour, which would help to ease overcrowding there. It would also add 4 trains per hour to the 7th Street/Mid-City subway between L’Enfant Plaza and Greenbelt. However, it would reduce the number of Blue Line trains headed for north Arlington and the Farragut Square/Metro Center side of downtown.

The Blue Line’s Franconia end would see the same number of trains as it does now, but some of those trains would head for Gallery Place instead of Farragut West. At Largo, existing headways would be maintained by rerouting the “new” Orange Line trains to Largo. The only station which would see reduced service is the little-used station at Arlington Cemetery.

Potential new Metro map showing rerouted trains as Yellow.

This change will affect riders on the southern end of the Blue Line. The diagram below shows how travel times could be affected. Trips toward the Farragut West area would take no more than 6 minutes longer — the additional wait time for a Blue train — but it is possible that the trains that remain on the current route would become much more crowded. Of course, passengers traveling to L’Enfant Plaza would save 9 minutes over the current one-seat ride (via Farragut), and 5 minutes over the transfer-to-Yellow (at Pentagon) ride.

Based on an analysis I conducted a few months ago, we can see an estimate of AM Peak volumes on the rail system. A look at those figures gives us an idea about the breakdown of passengers on the inbound Blue and Orange Lines. If we look at the two segments of track approaching Rosslyn on the Blue and Orange Lines, we can see that about 35% of the riders passing through Rosslyn are coming from the Blue Line, while the remaining 65% are on the Orange Line.

Since about 38% of trains (10 out of 26) are on the Blue Line and 62% are on the Orange Line, loads are probably pretty well balanced. The fact that around a quarter of Orange Line rush hour trains are 8-cars long means that the balance is even more appropriate.

This is probably one reason that Metro has not moved forward with its Blue/Yellow reshuffling proposal. However, once the Silver Line opens, Metro will not have much choice in the matter. In order to get enough Silver and Orange trains through Rosslyn, some Blue Line trains will certainly be rerouted via the Yellow Line bridge. A likely scenario for trains per hour at Rosslyn would be 10 Orange, 10 Silver, and 6 Blue.

On the other hand, It will take time for Silver Line ridership to build, so WMATA may delay any major restructuring until new ridership patterns have established themselves.

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 Capitol Hill. 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.