News today that the track circuit underneath the stalled Metro train in Monday’s crash failed:

The track circuit below the Washington Metro train that was rear-ended by another train this week didn’t work, U.S. transportation safety investigators found in a test.The circuit was supposed to relay information about the location of trains. The National Transportation Safety Board is investigating the June 22 accident in which nine people died. It was the worst in the 33-year-old Metro system’s history.

The circuit, part of Metro’s automated operations system, didn’t detect the presence of a test train investigators placed on it, the board said today in an e-mailed statement.

My speculation: In effect, the system didn’t know the stalled train was there.  Hence, it accelerated the back train and sent it at normal speed into a section of track the computer thought was clear.

The disturbing part would seem to be not the ATC system, but the fact that the sensor didn’t work.  ATC is a relatively new thing for railroads, but track sensors are not.  They work by sending a small current through each rail – tracks are divided into separate sections called blocks, and when a train’s wheels enter a block, the metal axle completes the circuit, sending a message to the control center on the train’s location.  This kind of technology has been in use for a long time.

The question is whether the failure is a part of the ATC system or the trac circuits.  Either way, it’s becoming more clear that the ignalling/control failure is primarily responsible here.

In the wake of Monday’s Red Line crash, DC’s various news outlets have uncovered all sorts of interesting factoids about Metro’s safety record and the implementation of various NTSB recommendations.  Speculation about the causes have run rampant, ranging from mildly informed theories to pure guesswork.  Some immediately blamed the now deceased operator, citing likely use of a cell phone.  DCist now reports via WTOP that’s not the case.

There’s also been a lot of discussion about the 1000 series rail cars and their crashworthiness.  At Greater Greater Washington, Matt Johnson has an excellent summary of Metro’s safety systems, particularly noting the design of the Automatic Train Control system as well as the track record of the 1000 series rail cars.   WCP gives you tips on how to avoid them, while the WMATA board agrees to move the cars to the middle of trainsets.

Public interest seems to focus on the 1000 series rail cars as the culprit, even though there’s no evidence that the cars themselves were the cause of the crash.  To me, the more interesting news to come out was the City Paper’s report that the NTSB found “anomalies” in the trackside equipment that’s part of the automatic train control system:

‘Anomalies’—that’s what federal investigators found in trackside electronic control equipment during testing yesterday, ’suggesting that computers might have sent one Red Line train crashing into another’ on Monday evening, WaPo writes. More from Lyndsey Layton, Maria Glod, and Lena H. Sun: ‘A senior Metro official knowledgeable about train operations said an internal report confirmed that the computer system appeared to have faltered.’ And that system, according to the NTSB’s Debbie Hersman, is ‘vital.’ Then there’s this: ‘The steel rails show evidence that McMillan activated the emergency brakes 300 to 400 feet before the pileup’—but she would have been traveling 59 mph. See also WTOP, NC8, WRC-TV, WUSA-TV, WTTG-TV, NYT, and Examiner, which notes that brake maintenance seems no longer to be an issue.

Again, it’s important to separate the two issues in this crash – the events that caused the crash itself, and the impact of the crash and the aftermath.  The former is about why this happened in the first place, the latter is about the crashworthiness of the cars.  Crashworthiness is important, without a doubt – but it’s also about keeping things as safe as possible after something has already gone wrong.

For that reason, the events that caused the crash itself are far more interesting to me.  We have evidence that the train was operating on Auto mode, that it was traveling quite fast (though eyewitness accounts tend to vary as to how fast – nevertheless, the damage shows a great deal of force was involved).  Given the slight curvature of the track, the speeds involved, and human reaction time to depress the Mushroom, it seems we can infer that the collision was unavoidable at that point – which would point to a very serious error with the Automatic Train Control system.

That’s where my interest is as the investigation unfolds.