"We recommended to WMATA (Washington's Metro Area Transit Authority) to either retrofit those cars or phase them out of the fleet, but they have not been able to do that and our recommendation was not addressed," said National Transportation Safety Board spokeswoman Debbie Hersman of a warning in 2006 at a press conference at the scene of the accident.

Two subway trains collided on the Red line between the Fort Totten and Tacoma stations in the northeastern part of Washington D.C., on June 22, 2009.  (Xinhua/Zhang Yan)

    The rescue efforts have been continuing through the night on Monday after a subway train operating on the Red Line slammed into another one parking on the track in northeastern Washington, D.C. during the afternoon rush hours.

    The accident resulted in nine deaths, the highest toll in the 33-year history of Washington's Metro Area Transit Authority, and also left 76 others injured.

    It remained unclear why the second train did not receive the stop signal when the first one was waiting on the track to drive in the metro station, or why the operator of the second train did not manually stop when she saw the blacked tracks ahead.

An injured man is carried away at the site where two subway trains collided on the Red line between the Fort Totten and Tacoma stations in the northeastern part of Washington D.C., on June 22, 2009. (Xinhua/Zhang Yan)

    Metro said it could take weeks or even months before the cause of the crash was determined.

    Metro spokesperson Lisa Farbstein told reporters that the subway car which struck the waiting one has been in service since 1976, among the oldest on the system. Although Metro officials were in the process of soliciting bids for replacements of cars like it, the cost remains an issue.

    "We don't have the money in place to replace them," said Farbstein.