The National Transportation Safety Board is investigating the incident which happened at the beginning of the Monday afternoon rush hour and led to the first fatality on Washington DC’s Metro system since a 2009 crash killed eight passengers and a train operator.
NTSB investigator Michael Flanigon told reporters that an electrical “arcing” involving the high-voltage third rail led a train to stop in a tunnel and quickly filled the tunnel with smoke.
An arcing occurs when electricity from the third rail comes into contact with another substance that conducts electricity, such as water.
While there was water in the tunnel, Mr Flanigon said the cause of the arcing was not yet known.
Witnesses described a chaotic scene aboard the train as passengers tried to escape the smoke, and many passengers left the train on their own before emergency responders arrived on the scene.
In addition to the woman who died, at least one other passenger was in critical condition at a local hospital. Eighty-four people were taken to hospital, most with smoke inhalation, authorities said.
The accident occurred on a Virginia-bound yellow line train that had just left L’Enfant Plaza, one of the system’s busiest stations. The Metrorail system connects Washington city centre with the Maryland and Virginia suburbs.