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.