WASHINGTON - INVESTIGATORS on Tuesday scoured the wreckage of two commuter trains hunting for clues to the worst subway accident in Washington's history which killed nine people and injured some 80 others.

Metro officials said they still had no clue why one train plowed into the back of another on an above-ground section of the system's Red Line just at the start of Monday's evening rush hour.

But the National Transportation Safety Board (NTSB), the federal agency that has taken over investigation of the crash, said the striking train was among the oldest in Metro's 33-year history, and that its operator had been driving the system's subway trains for just three months.

The driver, Jeanice McMillan, 42, whose train slammed into the other one with such force that it thrust two carriages from the front one up onto her train, died in the crash.

Preliminary investigations determined that the striking train was in 'automatic mode' and that the emergency brake had been depressed, indicating the operator may have tried to stop the train, the NTSB's Debbie Hersman said.

Last September, 25 people were killed when the conductor of a commuter train in Los Angeles was sending text messages on his mobile phone while at the controls.

NTSB had 19 investigators proceeding with a painstaking probe of Monday's accident which turned a routine commute into a nightmarish disaster scene. They would soon gain access to nine recorders on the newer, struck train - data Ms Hersman expected would shed light on what may have caused train 112 to plow into train 214 which was stopped on the same track.

Concerns have focused on the computerized signal system designed to prevent train collisions, and on the age of train 112, one of Metro's 1000-series trains delivered from 1975 to 1978.

As in many mass-transit systems, on-board computers control Metro trains' speed and braking, while another system monitors to see if there is a safe distance between trains, and automatically apply brakes if they get too close.

Thousands of government employees ride the Metro into work each day in a five-line rail system that carries about 800,000 people daily from the suburbs in the states of Maryland and Virginia. -- AFP