One Dead, Dozens Hospitalized After Heavy Smoke at D.C. Metro Tunnel

[DebJ]

7. Update: an arcing event between the third rail and power-supply cables

"Mike Flanigon, the railroad investigator for the National Transportation Safety Board, said at about 11:30 Monday night that the Virginia-bound train stopped about 800 feet beyond the station platform. About 1,000 feet on from there, "an arcing event" involving the third rail and power-supply cables was detected, which caused the smoke.

He said that arcing events happen when a current begins arcing to a conductor it's not designed to connect with -- it happens fairly regularly, and "you'll see a pop or a flash" while riding a train -- but in certain cases, "that arc can start feeding on itself, and it generates gases that are conductive." That produces a situation where, he said, "the worse it gets, the worse it gets."

He added that there was no fire on the train, nor did it derail. The investigation has just begun, he said."

http://liveblogwp.wtop.com/Event/Rail_impacted_after_LEnfant_Plaza_temporarily_closed