During rush hour, Yellow Line trains usually run between Huntington and Mount Vernon Square. But beginning in August, I noticed that many went all the way to U Street, two stops north of their usual ending point. The reason is a mechanical malfunction with a track at Mount Vernon Square that lets trains change direction, and the issue highlights just how important that track is to the entire system.

Photo by the author.

In technical lingo, the problem is called a “bobbing track circuit.” The issue is typically considered more of a nuisance than an actual danger, but it’s been preventing rush hour Yellow Line trains from accessing their usual turnaround point (the Mount Vernon pocket) for several weeks.

According to WMATA operations guru Stephen Repetski, a track circuit is described as “bobbing” when it erroneously flips back and forth between registering that a train is occupying its block when it might not actually be. This may occur because the circuit is too sensitive, or it may be due to old equipment that needs to be replaced.

A bobbing track circuit can cause trains to lose their speed commands. This forces the operator to stop and wait for permission from the Rail Operations Control Center to proceed to the next signal block, which they must do at a reduced speed. This significantly delays train operations.

WMATA signaling infrastructure. Photo by Matt’ Johnson on Flickr.

Usually, rush hour Yellow Line trains reverse direction at Mount Vernon Square because there’s a pocket track there—a short third track located between the two main tracks, which allows trains to reverse direction without blocking trains on the mainline. In normal rush hour operation, Yellow Line trains use this pocket track to short turn, e.g. to turn around without running all the way to the end of the line at Greenbelt.

Since Yellow Line trains currently have to turn around using the crossover at U Street* (or run all the way out to Greenbelt), fewer trains are operating on the Green/Yellow Lines every rush hour. This is due to the fact that when trains have to short turn without the use of a pocket track, they cause delays by blocking trains in both directions, as Matt Johnson detailed in a 2010 post on the issue.

*There are crossovers that allow trains to turn around between every station from Gallery Place to Fort Totten, but Metro seems to have chosen U Street as the temporary turnaround point.

Trains that have to short turn without the use of a pocket impede through trains in both directions. Graphic by the author.

However, a pocket between the mainline tracks allows short-turning trains to be taken off the mainline while reversing direction, thus allowing through trains to proceed unobstructed. Thus, when rush hour Yellow Line trains turn back toward Huntington using the pocket track at Mount Vernon Square, they are allowing through trains (e.g. Green Line trains) to pass unimpeded on the mainline while the reversal procedure is carried out.

A pocket allows trains to reverse direction without blocking the mainline tracks. Graphic by the author.

This temporary “extension” of Yellow Line service did get a few Greater Greater Washington contributors thinking: could Metro ever permanently extend the Yellow Line past Mount Vernon Square during rush hour? Doing so would provide booming stations like Shaw, U Street, and Columbia Heights with increased service and a direct connection to Virginia at all hours.

There are, however, several financial and logistical problems standing in the way.

The obstacles: lack of a pocket track, railcars, and funding

After 15 years of short-turning Yellow Line trains at Mount Vernon Square at all hours, Metro began running off-peak Yellow Line trains all the way to Fort Totten in December 2006. The service started as a pilot program, but was quickly made permanent due to high demand.

However, there are three major obstacles preventing Yellow Line trains from running to Fort Totten during peak hours: there’s no a pocket track at Fort Totten, Metro only has so many trains, and Metro doesn’t have the money (Matt explored these factors in depth in his 2010 post).

An above-ground pocket track north of Silver Spring. Image via Wikimedia Commons.

Turning some trains around at pocket tracks (like the one at Mount Vernon Square) rather than running them all the way to the end of the line allows more trains to run on the given section of the Metro system because it keeps terminal stations like Greenbelt from getting overwhelmed. For example, 50% of rush hour Red Line trains only run between Grosvenor-Strathmore and Silver Spring (rather than all the way from Shady Grove to Glenmont), using the pocket tracks at these stations to reverse and head back in the other direction. This means that Shady Grove and Glenmont only have to handle turning around 13 trains per hour, rather than the maximum of 26. There is excess rush hour capacity on the Green/Yellow Line tracks north of Mount Vernon Square, as you can see in Matt Johnson’s graphic below, and extending the Yellow Line to Fort Totten would take advantage of it (as the four hourly Rush+ Yellow Line trains to Greenbelt already do).

Metro service vs. capacity. Graphic by Matt Johnson.

But without building a pocket track somewhere between Mount Vernon Square and Greenbelt (such as at Fort Totten), it is impossible to take full advantage of this capacity without overwhelming operations at the busy Green Line terminal. Unfortunately, there are significant obstacles standing in the way of this. In addition to the cost of the pocket track itself, Metro would have to pay for additional rolling stock in order to operate the extended Yellow Line service full time. As Matt explains, the increased service would require 30 additional railcars at a cost of approximately $100 million, plus $3 million per year in operating costs. The new railcars would have to come from Metro’s order for the 8000 series, which aren’t expected to begin delivery until at least 2023. Is it possible to build an extra pocket track between Mount Vernon Square and Greenbelt? And how come one didn’t go in in the first place? I’ll take a look at that in a post next week. Correction: This post initially said there are crossovers at every station between Gallery Place and Fort Totten. That isn’t the case, and given where the crossovers are, U Street makes sense for the current Yellow Line issue.