WMATA's recent apology and the real problem that isn't their fault: lack of redundancy, bad design

Image of the interior of a WMATA train station from Creative Uncut.

The recent apology by WMATA GM Richard Sarles was sparked by a cable failure on the Red Line earlier last week ("Metro chief Richard Sarles apologizes to Red Line riders after second day of major delays," Post) which caused big delays. The cable was affixed to the tunnel roof, it came loose, dangling over the tracks, and re-securing the cable took a lot of time, because the height of the tunnel is higher than typical ladders.

This post is sparked by a letter by Peter Kolker in yesterday's Post "Make Metro managers accountable" proposing that pay for top managers be docked when the system doesn't meet performance criteria, and closes with this:

There are myriad excuses for Metro's problems, but New York's system is 70 years older than ours and seems to run much more smoothly.  Maybe accountability has something to do with it.  Let's give it a try.

While accountability is an issue, it has little to do with the problems inherent in the WMATA system, which are the result of the basic design of the system. 

One of the books I came across in college was Complex Organizations: A Critical Essay by Charles Perrow, an organizational sociologist.  That's the first time I was introduced to the concept of "redundancy," about which in a later book Normal Accidents: Living with High-Risk Technologies Perrow wrote about in much greater detail.

Redundant sub-systems provide a backup, an alternate way to control a process or accomplish a task, that will work in the event that the primary method fails. This avoids the "single-point" failure modes.

Interactive complexity is a system in which two or more discrete failures can interact in unexpected ways is described as "interactively complex." In many cases, these unexpected interactions can affect supposedly redundant sub-systems. A sufficiently complex system can be expected to have many such unanticipated failure mode interactions, making it vulnerable to normal accidents.

The sub-components of a tightly coupled system have prompt and major impacts on each other. If what happens in one part has little impact on another part, or if everything happens slowly (in particular, slowly on the scale of human thinking times), the system is not described as "tightly coupled." Tight coupling also raises the odds that operator intervention will make things worse, since the true nature of the problem may well not be understood correctly.

Normal accidents ... a synonym for "inevitable accidents." This categorization is based on a combination of features of such systems: interactive complexity and tight coupling. Normal accidents in a particular system may be common or rare ("It is normal for us to die, but we only do it once."), but the system's characteristics make it inherently vulnerable to such accidents, hence their description as "normal." 

The two track per line WMATA system lacks redundancy.  When problems occur, they cascade, because the system is "tightly coupled." 

The initial problem with the cable wasn't a fault of redundancy exactly, but design. The vaulted tunnels look cool but weren't designed for practical maintenance in terms of needing special equipment to reach beyond normal ladder height. So fixing the problem took extra-normal resources and time.  Note that other subway systems in North America don't have vaulted tunnels, but rectangular tunnels (such tunnels are also easier to light).

The dangling cable problem was accentuated by the reality that the WMATA system wasn't built with any significant level of redundancy. Even though the system was and is expected to reach over 1 million daily riders, each line was constructed to have only two tracks, one for each direction, although there are some switching tracks here and there across the system, enabling trains to switch from one track to the other.

If there is a problem with one track, it goes out of commission and shuts down, leaving only one track to use for two tracks worth of trains. This reduces the speed and throughput of the system by more than 100% because running two directions on one track is more difficult, less safe, and less efficient than running trains in one direction. 

By comparison, the Wikipedia page for station configurations of the NYC Subway system has 8 different diagrams depending on whether the station has 2-, 3- or 4-tracks, and whether the station has express and/or local service.


If the WMATA system had been built with three tracks, not only could express services run from certain points, but there would be "back up" or slack capacity and redundant systems, so that a problem with one track wouldn't bring the system to a halt.

The lack of redundancy becomes more pronounced as the train equipment ages and is subject to more frequent breakdown. A "broken down" train takes a track out of commission. More frequent train breakdowns lead to more systemic failures or "normal" or "inevitable" accidents.

Sadly, the Silver Line extension could have been built with a third track although not being able to extend a third track into the core of the system means that express trains wouldn't have been a possibility, but it would have added redundancy, at least for that section.

In short, expect a lot more apologies, for things that WMATA operations personnel have almost no control over (cf. "Learning From the Superstorm," New York Times Magazine, which describes how NYC Transit personnel were able to use their deep knowledge of the system to restore system comparatively quickly after Superstorm Sandy).

Fundamentally, the  problem is one of engineering and design and is uncorrectable without rebuilding the entire system so that each line has at least three tracks.

Note also that the NYC Transit system had its own period of failure in the 1970s and 1980s in response to years of underfunding maintenance and upgrades and that by having many more lines than 5, there is a quantum level difference in the redundancy built into that system as it is not just having more than two tracks on a single line, it's also having multiple lines.