June 25 (Bloomberg) -- 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.

The system, when working properly, should slow a train that is approaching another. In the crash, which injured about 80 people, one train heading south on Metro’s Red Line plowed into the rear of another train that was stopped on the same track.

To contact the reporter on this story: Angela Greiling Keane in Washington at agreilingkea@bloomberg.net

Last Updated: June 25, 2009 17:36 EDT