Metro's growth in ridership has never been pain free, but overcrowding on the Orange Line is reaching absurd levels. WMATA has already lengthened many trains to 8 cars, which is the length of the platforms. A shortage of rollingstock and power substation issues are preventing more trains from being lengthened, but even so, the day is coming when nothing can be done but add trains.
This unfortunately, can only happen
at the expense of the Blue Line.
Metro trains can operate 90 seconds apart, but require 135 seconds of separation at locations where switches must be realigned between trains. In this case, the bottleneck is the "Rosslyn Portal," the area of tunnel just west of Rosslyn Station, where the Blue and Orange Lines diverge.
Planners at WMATA have gotten a little extra capacity out of the system by running two consecutive Orange Line trains followed by one Blue Line train. This way, the switches don't need to be realigned as much--and that saves 45 seconds. This is possible because for every 2 Blue trains, WMATA runs 3 Orange trains. Still, the throughput is limited.
Since pictures are worth more than words, I'll just demonstrate by referring to the diagram below. *If you don't understand something, leave a comment, and I'll try to address it.*
Based on the 135 second headway, WMATA can run 5 trains through a given segment of track every 12 minutes. Each the diagrams below represents a 12 minute interval during rush hour. Each of the lines on the diagram represents a train in each direction. Therefore, a trackway with two lines (like between Navy Yard and Anacostia) represents a headway of 6 minutes--12/2.
The first diagram represents WMATA's current service pattern during rush hours.
*In the first version of my maps, I erroneously showed the Yellow Line going all the way to Fort Totten. During rush periods, the Yellow Line terminates at Mount Vernon Square. The corrected map is below.
The chief limitation for the Orange Line, as you can see here is the 4 minute headway on the Vienna-Rosslyn segment. Adding one train would reduce headways to 3 minutes and would add a capacity of 1000-1400 passengers for every 12 minute period. Any additional capacity is sorely needed, but the segment of track between Rosslyn and Stadium Armory is essentially at capacity.
Hence the so-called "
Blue Line Split."
Under this proposal, approximately every other Blue Line train leaving Franconia-Springfield would follow the Yellow Line across the Potomac, and up to Greenbelt. While this would shave about 8 minutes off of a trip between Franconia and L'Enfant, it would decrease the frequency of trains running directly to Rosslyn, Farragut, and Metro Center from Franconia. According to Metro reports, however, trips from the Alexandria/southern Fairfax stations to Farragut and Metro Center have been decreasing (by about 4%) while trips to the east side of Downtown (L'Enfant, Gallery Place) have been increasing (by 11%).
Here's what WMATA is proposing:
*In the first version of my maps, I erroneously showed the Yellow Line going all the way to Fort Totten. During rush periods, the Yellow Line terminates at Mount Vernon Square. The corrected map is below.
Under this scenario, all stations maintain their current headways or increase them except for Arlington Cemetery. To offset the Blue Line trains to Greenbelt on the east side of the system, the new Orange Line train that will be running in place of the Blue Line train will also run to Largo.
While the Blue Line via Rosslyn is certainly less patronized, there could still be potential for overcrowding. This could be alleviated by making sure that all Blues via Rosslyn are 8 cars (which hold about 400 more passengers than a 6-car train).
Still, some are understandably upset. But there is a greater good to think about. All they need to do is get to the Metro station 6 minutes earlier, and they'll get to work 6 minutes earlier. There is no trip time impact to Blue Line trains operating via Rosslyn. Of course, that's assuming that normal operations are underway. Since the Blue Line Split will increase efficiency and reliability at the Rosslyn Portal, riders via Rosslyn on both Blue and Orange Lines will experience fewer delays, and therefore an increased trip time on average (not considering wait times).
To analyze the travel time cost to riders, I created an additional schematic (I've spent several hours this week working on this post--that's what having a cold is good for). The schematic on the left side of the diagram shows the current travel times. Assuming someone wanted to leave Franconia-Springfield at 7:30, they would take train "A." Were they to miss that train, the next one (B) would leave 6 minutes later at 7:36. Since trains take the same routing, B would arrive at each destination 6 minutes after A. Passengers wishing to travel to L'Enfant would save 5 minutes by transferring to the Yellow Line at King Street or Pentagon.
On the right side of the diagram, I've shown projected travel times (based on current station-station travel times) with the Blue Line Split. Since approximately every other Blue train at rush hour would follow the Yellow Line, I've assumed that train A would be the re-routed train. It has the same departure time from Franconia as train A does under the current travel time scenario.
Passengers traveling to L'Enfant now save 9 minutes over the one-seat ride, and 5 minutes over the transfer-to-Yellow ride. Passengers traveling to Farragut West and Metro Center would still get to work quicker by waiting 6 additional minutes at Franconia for train B, but they would only be saving 3 minutes over train A. But their trip is not actually longer, just their wait. Since trains (even Metro trains) run on schedules--and remember, this rerouting will help on-time performance--the riders in question can just show up at Franconia (in the AM) or Farragut (in the PM) 6 minutes earlier or later and get to work in the same time.
The only people inconvenienced by this plan are people traveling to Arlington Cemetery during rush hours. Most of them are tourists, and can use their additional 6 minutes of waiting to puzzle over the Metro map or stand at the top of an escalator.
Therefore, I judge the direct cost to Franconia->Farragut riders as 6 minutes each way. Some will not like this. They are
already complaining to Dr. Gridlock and Metro, but the time savings for everyone else in the system is quite appreciable.

Some have also contended that this scenario will cause confusion. Staff at WMATA have proposed dubbing the reroute, which would only occur during rush hours,
the Brown Line. David over at Greater Greater Washington disagrees. He thinks dubbing it the Yellow Line
makes the most sense, and I agree. For simplicity's sake, on the diagram showing the re-route, I have colored it light blue.
This proposal becomes even more essential (and probably assured) once the Silver Line to Dulles comes on line. With trains from Tysons and Loudoun County sharing tracks with the Orange Line between East Falls Church and Stadium-Armory, service will need to be reduced at WFC, Dunn Loring, and Vienna (see below).
These cuts would be less if the Blue Line had a
separated subway along M Street, as WMATA is now proposing. Funding for this massive project has not been identified, but in my opinion is more likely under an Obama Presidency.
I also think that the Silver Line will eventually need it's own tunnel downtown. After all, as popular as the Orange Line is currently, the Silver Line will serve much more popular destinations. There's not much at Vienna, while the four stations in Tysons will be in the largest job center in the entire state of Virginia. Additionally, Silver trains will run much further into the suburbs than Orange Line trains--so they might be more popular with commuters. At the same time, I don't see the popularity of Vienna's parking garages dropping off.
*In the first version of my maps, I erroneously showed the Yellow Line going all the way to Fort Totten. During rush periods, the Yellow Line terminates at Mount Vernon Square. The corrected map is below.

Anyone have any thoughts? Questions?