The discussion of the underlying suspected cause of the crash, " the susceptibility of the audio frequency track circuit to errant signals" is more than a little confusing. The upshot of this phrase is that a track circuit in the area of the accident lost the ability to detect stationary train 214 and consequently train 112 did not receive an automated command to slow down or stop.
The concern is that routine track adjustments may have caused changes in equipment designed to automatically detect trains.
The NTSB has recommended that WMATA (Metro) work in conjunction with its equipment providers to look at train control systems more closely.
The "urgent" recommendation is for Metro to examine its track circuits for problems which could adversely effect the ability to detect trains.