Image from Google street view.

Yesterday afternoon, a construction accident caused the collapse of the pedestrian bridge over the Green Line and CSX/MARC tracks in Berwyn Heights. The debris blocked the line between College Park and Greenbelt, disrupting many commutes. But why were there ripples as far away as Alexandria?

Since the Green Line between College Park and Branch Avenue was unaffected, it’s hard to comprehend how the bridge collapse would affect any commuters other than those going to Greenbelt. But if you consider how Metro uses its trains during peak hours, it’s clear why the incident had such far-reaching consequences.

There were two major reasons that the collapse affected trips on the Green and Yellow Lines. The first is that Metro’s largest rail yard is at Greenbelt. Since the collapse happened during midday, many of the trains that would have soon been heading downtown to collect commuters were trapped there.

The second issue is related to the first. With fewer trains, and because Metro decided to extend all Yellow Line trains to College Park, there simply weren’t enough trains to provide the regular headway.

Trains couldn’t leave Greenbelt

During rush hours, Metro runs most of the cars in its fleet. But at the end of rush hour, Metro sends many railcars back into the yard.

For example, during rush hours, Metro has about 17 trains in service on the Green Line. But after rush hour ends, the number of trains drops to about 9.

Yesterday, just before 3:00, the bridge collapsed just as Metro was about to transition from a midday to a rush hour schedule. Any trains that were north of the collapse were stuck there, including 60 railcars in the Greenbelt Yard, according to spokesperson Dan Stessel.

Those 60 railcars could have made up 10 6-car trains, which would’ve been assigned to both the Green and Yellow Lines. Suddenly, though, they were unavailable.

The Green Line also has a rail yard at the southern end at Branch Avenue. The Yellow and Blue lines share the Alexandria Yard, near King Street. But Metro doesn’t keep enough cars in those yards to run full service.

Frequency, run time, and the number of trains are all related

Most people probably never think much about all the details that go into scheduling, but there’s a basic equation that balances the frequency, run time, and the number of vehicles needed to run a given service.

As discussed above, the bridge collapse reduced the number of available trains. Obviously that will have an impact on the schedule. But the other thing that had an impact was extending the Yellow Line.

Figuring out how many trains (or buses) it takes to run a service is essentially as simple as dividing the cycle time by the desired headway.

For example, during the midday period, the Yellow Line runs from Huntington to Fort Totten. It takes a train 36 minutes to get from Huntington to Fort Totten, and 36 minutes to get back. If we assume a recovery/layover time of three minutes on either end, that gives us a cycle time of 84 minutes. That’s how long it takes one train to run the route and be ready to go again.

Now, during this period, the Yellow Line runs every 12 minutes. That’s the headway.

If we divide the cycle time (84 minutes) by the desired headway (12 minutes), we discover that we’d need seven trains.

If we change one of those variables, either of the other two (or both) variables must change as well. For example, if we want to double the frequency so we have a train every six minutes, we’d need 14 trains, double what we needed before.

Metro does this exact thing during peak hours. They double the frequency of the Yellow Line. But they also change a different variable: cycle time.

That’s because during peak hours, the Yellow Line (not counting rush plus) only runs from Huntington to Mount Vernon Square. The cycle time is shorter (56 minutes), which means it only requires 10 trains to operate (instead of the 14 needed to run to Fort Totten).

Of course, the primary reason that WMATA doesn’t run to Fort Totten during rush hour is because there’s no pocket track there, and trains come too frequently (every 3 minutes) to have Yellow trains turn back on the mainline.

So what about yesterday?

What happened yesterday was a combination of changes to all three variables.

Because several trains were trapped north of the bridge, the number of available trains was lower than usual.

To help alleviate delay to customers headed for Greenbelt, and probably to deal with frequency issues north of Mount Vernon Square, Metro extended many or all (that’s not entirely clear) Yellow Line trains to College Park. That lengthened the cycle time to about 96 minutes.

If Metro only had seven trains, a longer a cycle time of 96 minutes, would mean the headway on the line would become 13.7 minutes (instead of the usual six).

An example of how Yellow Line runtime and number of trains affects headway. Graphic by the author.

Now, Metro probably had one or two trains in Alexandria that they were able to put into service, which would shorten that headway a bit. I even saw a report on twitter that the #newtrain was switched over to the Yellow Line during the evening rush hour.

Dan Stessel indicated that other than an initial delay while the damage was being assessed, the Green and Yellow Lines ran close to on time. However I did see many tweets bemoaning extra long waits.

In addition to the changes to rail service, Metro put 20 buses into service to run the bus bridge between College Park and Greenbelt.

Hopefully this helps explain a bit about how the length of a train’s round trip, along with how many trains (or buses) there are, affect how frequently they run. These examples are specific to what happened yesterday in Berwyn Heights, but the variables apply to the entire system.

For example, bus lanes and transit signal priority are ways planners try to shorten the cycle time, which allows more frequency with the same number of vehicles.