The National Transportation Safety Board has put up a second advisory today on its web site regarding the most recent findings of its investigation of the June 22 Red Line crash. In it, we learn for the first time that the train control system track circuit where the accident occurred — the same one that investigators initially found to be not functioning properly after testing it during the first couple of days after the crash — was replaced by WMATA maintenance crews on June 17, five days before the accident. From the advisory:

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.

No conclusions from NTSB on this yet, of course, but given the timeline and the recorded data they mention, it certainly suggests the strong possibility that this was a faulty circuit which failed almost immediately after it was installed.

Also of note from the advisory: On the weekend of July 18, investigators are planning to conduct sight distance tests using trains consisting of similar cars to those involved in the accident. The tests are designed to establish when the struck train would have been visible by the operator of the striking train.