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The federal safety board investigating last month's deadly Metro crash is focusing on two pieces of electrical equipment - more than 30 years old - that appear to offer the clearest clues yet about why the train protection system failed, allowing a Red Line train to ram into one ahead of it north of the Fort Totten station.

In an update issued Wednesday, the National Transportation Safety Board said investigators have been concentrating on two track circuit modules at the accident site. They were due to be replaced this year as part of an overall upgrade of Metro track circuits.

Since the June 22 accident, investigators have been trying to determine why the track circuit at the accident site was failing to properly detect the presence of a train. Investigators have replaced different components, but the circuit continued to malfunction, showing "anomalies," officials have said. After the modules were replaced twice, the track circuit "did not display these anomalies," the NTSB said. Track circuits detect the presence of a train using audio frequencies transmitted between the train and the steel rails.

Like most of the electrical components in the track circuit system, the modules are original equipment from the mid-1970s. They were manufactured by General Railway Signal, which is now owned by Alstom. The modules are housed in a control room near the Fort Totten station. About the size of an old computer monitor, the modules each contain 20 circuit boards that receive and transmit signals from the track.

The safety board said each of the components is being tested extensively to determine the effect of their performance on the train control system, and how any change or degradation in the component might affect the system. Metro crews had replaced other critical components, known as impedance bonds, at the site before the accident, including five days before the crash.

As part of the process for replacing the bonds, adjustments were made to track circuit signal strength, the safety board said. According to the update, "The investigation is evaluating any effect that these track circuit adjustments may have had on the performance of track circuit modules located at Fort Totten."

A safety board spokesperson said the investigation is "still very much in the fact-finding stage" and said Wednesday's update was a "more detailed and thorough explanation of what investigators are doing on scene."

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