WASHINGTON - INVESTIGATORS probing the deadliest accident in the history of the Washington Metro have found 'anomalies' in a circuit in the track where a train slammed into the back of another this week, killing nine people.

Five of six circuits in the section of track where the accident occurred at the start of the evening rush hour on Monday 'operated per standard' when investigators from the National Transportation Safety Board (NTSB) ran a device that simulates train wheels over the rails, board official Deborah Hersman said.

'One of the circuits, however, has some anomalies. That's the circuit that our investigators are paying close attention to now,' Ms Hersman told reporters at a news conference Wednesday.

The circuits send speed commands to trains and tell them when to stop or proceed, Hersman said.

An NTSB team that walked the stretch of track between two commuter stations in northeast Washington where the accident occurred found indications that the driver of the striking train, 42-year-old Jeanice McMillan, tried to stop her train from plowing into the back of the stationary train.

There was 'some bluing of the rails approximately 300-400 feet prior to the point of collision,' said Ms Hersman.

'Bluing indicates that there was some emergency braking that might have taken place,' she added.

Earlier probe results have shown that the emergency brake on the train McMillan was operating had been depressed.

Two electronic brake control units (EBCUs) have been extricated from the mangled wreckage of the first two cars of the train operated by McMillan and would be 'bench-tested' on Thursday, Ms Hersman said.

Investigators have already reviewed the EBCUs taken from the last four cars of the six-car striking train and found no defects, she told reporters. -- AFP