The NBC story reporting the fatality is here. The embedded video was made earlier. Chech nbcwashington and wjla for updates. (WJLA had not reported the fatality as of 7:25 PM but NBC4 had.) WJLA's latest coverage was here. (As of 7:38 PM WJLA reported the death, but its sister News Channel 8 reported it the same time as NBC4.) Vox and CNN did not have reports yet as of 7:41 PM, despite the conceivable implications of the situation.
Update: Jan 13
The best information, preliminary, from the NTSB is that the smoke came from a smoldering fire resulting from a short cicuit involving the third rail, about 1000 feet further toward the end of the tunnel that opens to the 14th Street Bridge for the Yellow Line. The train stopped 200 feet south of the station going toward the bridge. It is shocking the smoke built up so quickly and that Metro did not get the passengers out of the tunnel quickly, with only 200 feet to walk. There were many comments on the Washington Post story, one of which envisioned how Metro would respond to a terrorist attack in a tunnel (which ABC Nightline had simulated in 1999, before 9/11, on San Francisco BART with anthrax, see "cf blog" Sept. 20, 2013)
The liability insurance company and its lawyers are going to be busy making litigation settlements.
Update: Jan 17
NTSB preliminary report on cable arcing that caused fire in tunnel here; current not turned off for almost 40 minutes, leading to smoke. DC Fire report, emphasizes lack of radio in L'Enfant Plaza station.
Will Metro weekend shutdowns increase even more? It seems like all this weekend track work and maintenance has been unable to maintain the system.