Metro FAQ: Why does Metro run express trains in one direction during single-tracking?
Twice this week, attempted suicides have caused single-tracking during busy times. In both cases, Metro sent trains in one direction express through the single-tracking zone. Why would it do this?
The Metro system has two tracks on each line. There are interlockings, where trains can change tracks, every so often. Whenever they single-track, the track that's open has to take turns carrying trains in each direction. Generally, Metro will send a few trains going the same direction through at once and then reverse the track.
Of course, the reason that delays accrue quickly is because it can take a train 4 or 5 minutes to clear the single-tracking section. While a train goes through in one direction, trains going the other direction are holding (and stacking up) on the other end.
On Wednesday, when a person jumped in front of a train at Rhode Island Avenue just before the start of the evening rush hour, Metro sent outbound Red Line trains through normally. But inbound trains were run through without stopping. By doing this, Metro mitigated the delay.
This is because Metro trains lose about a minute stopping at each station. The train is generally stopped only for about 30 seconds, but there's also the time lost to deceleration and acceleration.
The normal travel time between Union Station and Rhode Island Avenue is about 4 minutes. If Metro sends 2 trains through in the same direction (with the second about 90 seconds behind), the time it takes Metro to reverse the single-tracked section is about 6-7 minutes.
That means the cycle time is about 12-14 minutes.
But if inbound trains skip intermediate stops, they can save time, which reduces the cycle time. If outbound trains take 6 minutes to clear, but inbound trains can do it in 3 or 4 minutes, the cycle time is reduced from 12-14 to 9-10 minutes.
It does create additional delay for anyone boarding or alighting at those stations, at least those going in the direction skipped. But anyone going through the zone saves time.
However, Thursday's incident at Waterfront was more problematic. Metro sent northbound trains through from Navy Yard to Archives without stopping. That meant that northbound Green Line trains skipped L'Enfant Plaza, and riders couldn't transfer to or from the Blue or Orange lines. A passenger at Waterfront who wanted to catch the Orange Line would first have to take a Branch Avenue train to Navy Yard, then a Greenbelt train to Archives, then a Branch Avenue or Huntington train back to L'Enfant Plaza.
Metro did try to mitigate problems somewhat by running all Yellow Line trains to Greenbelt, and those trains did stop at L'Enfant. But it was still a mess. Of course, given the delays, it likely would have been a mess no matter what Metro did.
But aside from the L'Enfant Plaza issues, skipping stations in one direction looks like a promising way to mitigate delays during an unplanned service disruption.
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