Most of you have heard probably heard about Metro’s proposed plan for a loop subway around the core. The idea is to “reduce future crowding on Metrorail, provide enough capacity for future development, and expand the reach of transit in the region” However, I think that it’s only a marginal improvement that would only somewhat reduce the central cause of Metro’s core capacity issue: multiple lines sharing track.
As you can see on the map, the Orange Line shares track with the Silver Line and the Blue Line, the latter of which shares track with the Yellow Line. The Yellow Line in turn shares track with the Green Line. The Red Line is the only line is operationally independent. This series of joint trackage causes a succession of operational and logistical hassles and limits that wouldn’t be an issue if the lines were more independent.
On any section of track, Metro can only run 26 trains per hour in one direction. So on the Red Line during rush hour WMATA runs trains about every three minutes each way between Silver Spring and Grosvenor, hitting that 26 train per hour limit. Since metro doesn’t currently run every Red Line train all the way from Shady Grove to Glenmont, there is some unused track capacity north of Grosvenor and Silver Spring, respectability. Taking advantage of this would simply be a matter of purchasing more train cars, hiring more drivers, and maybe a few upgrades to the power system.
On the other lines, however, the interlining of different rail lines causes more complication that just a few purchases and upgrades.
This May, I was heading to a National’s Game, which is at the Navy Yard stop on the Green Line. Heading from Virginia, I needed to transfer at the L’Enfant Plaza station. When I got there, a Blue Line Train had knocked out track power at Pentagon. This wasn’t resolved quickly and there was a backup of trains both Blue and Yellow which couldn’t end get past Arlington Cemetery and L’Enfant, respectfully. So while the Green Line was unaffected directly, I couldn’t get on a train since there was a Yellow Line one in the way that couldn’t get to Pentagon. The backup on the Blue Line started to affect the Orange line as well, as trains couldn’t advance past rosslyn either. So a Blue Line issue ended up backing up Yellow, Orange, and Green too. As it is separate, only the Red Line was unaffected by this. I ended up walking.
Let’s take a took at the poor Yellow Line, which only has two stations and a bridge to call its own. The Yellow Line shares track with the Blue Line in Virginia and then continues onto its destination Green Line station of Mt. Vernon Square, Fort Totten, or Greenbelt, depending on the time of day, track work, or which trainyard the train you are riding on “belongs” to.
Remember that since The Yellow Line shares track with the Blue and Green Lines, it is limited in the number of Yellow trains it can run. For example, let’s say Metro decided that to meet demand for Green Line Commuters it needed to run 15 trains per hour during rush. That would mean there would only be eleven slots for yellow line trains. This is even before we get to the limits that the blue line causes. Metro currently runs seven trains per hour on the Blue Line between Franconia and Largo. In this scenario with Metro deciding on 15 Green Line Trains assuming the same number of Blue Line Trains as currently, there would be unused capacity south of Pentagon. With a 26 train per hour capacity, and eleven yellow line trains and seven blue line trains, there would be eight train slots that metro could not use.
Thankfully, at the moment there aren’t big capacity issues on the Yellow and Green lines that force Metro to make tradeoffs like that. However, on the Virginia side of the Orange and Blue Lines, there are capacity issues and the Silver Line will only make those tradeoffs more sharp. The Orange, Blue, and soon-to-be Silver lines share track between Rosslyn and Stadium-Armory. Just like any other piece of track, the 26 trains per hour limit applies and it create’s Rosslyn’s Orange Crush. There’s serious demand for taking metro from the parts of Virginia served by the the Orange Line. There’s more people than there are trains to carry them. Yet, west of Rosslyn, they haven’t hit the 26 train per hour limit. Metro is already are already running 26 trains per hour from between Rosslyn and Stadium-Armory. 19 Orange and 7 Blue. That means there’s room on the west of Rosslyn for at least seven more trains. They can’t add them without further deducting blue line service and Metro already did this by sending three trains over the Yellow Line bridge to the Green Line in order to add three more Orange Trains. They expect to continue this with the Silver Line:
This gets to the crux of why interlining is a bad idea in the long term. It may be a cheaper way to add capacity in the short term. But, at some point with growth you reach a point where growth in ridership of one line starts to detach from the service for the line that one shares track with. It results in tradeoffs that are no where near the optimum outcome and force even more construction to fix a problem that should have been fixed in the first place by the original expansion.
Which is why this loop is good short-term but bad long term. Short-term the Green, like the Red Line, would operate independently and could run many more trains than it does today. Those express tracks would take some pressure off the regular Orange/Silver Lines. Put you would replace five lines that would be operationally chained but somewhat separate into two groups (Green/Yellow/Blue and Orange/Blue/Silver) with four lines that would be attached at the hip.
The number of Blue/Yellow trains would be limited by the number of Express Orange/Silver. Ditto, the number of Express Orange/Silver would be limited by the number of Blue/Yellow trains. If the loop is bi-directional (which brings in its own set of problems), you could get saturate the 52 bi-directional capacity with both lines, but that’s ONLY if all the trains go into the loop and out of the loop. Were there ever to be another branch off the loop, and expansions the other reason why you add core capacity, then these limits would still be there, subtracting from the number of slots for Blue/Yellow and Express Orange/Silver.
But then again, I’m an idealist. I’d love my grandchildren to have more lines to more places than just a loop, and to be able to not wait for a train to get there.