Dr. Gridlock has a couple of posts on the NTSB’s recent discovery of the failure of the ATO system to detect that Red line train:

As previously reported, initial testing showed that when the test train was stopped at the same location as the train that was struck in the accident, the train control system lost detection of the test train. Additionally, in subsequent testing over the weekend the train detection system intermittently failed; data is currently being collected to further analyze each component in the train detection system. Investigators are reviewing recorded track circuit data for each test configuration.

Maintenance records show that an impedence bond for the track circuit where the accident occurred was replaced on June 17th, five days before the accident. After a post- accident review of recorded track circuit data, WMATA reported to the NTSB that the track circuit periodically lost its ability to detect trains after June 17th; the NTSB is reviewing documentation on the performance of that track circuit both before and after the June 17th replacement.

Bold is mine.

John Catoe’s statement, with an excerpt:

Our testing has resulted in our being able to replicate the problem, but not isolate the specific cause. We know the problem is in a track circuit. We could just replace the parts, but we need to understand what caused it. You don’t just change the parts. We must find the cause.

We have conducted computerized analytical tests, which the NTSB has referenced as “track circuit data.” The data establishes a profile of what’s taking place electronically in the rail system. These tests are normally conducted monthly. What we found during a special review of the data after the accident was that the track circuit periodically lost its ability to detect trains. This is not an issue that would have been easily detectable to controllers in our operations control center. What the analytical profile showed was that the track circuit would fail to detect a train only for a few seconds and then it appeared to be working again. This happened after we had replaced an “impedence” or “weezie bond” for the track circuit for where the accident occurred. The device communicates information such as speed and distance between the tracks, trains and operations control center. The device was replaced as part of Metro’s normal track rehabilitation program. We are now running analytical reports on the rail system daily instead of monthly and system wide. We have found no other similar issues with track circuits in the system.

Again, bold is mine.

So, the question now is to determine just how periodic this detection failure was – how often, and for what time frame.  “A few seconds” could mean 1 or 2 seconds, or it could mean more.  It would seem that the longer such a glitch occurs, the more likely it is that a collision could take place.  Without knowing what kind of data Metro operators have before them on an updated basis, it’s impossible to tell if such a glitch should have been detected by the operators.

We don’t have the full story yet, but the confluence of circumstances is starting to build.