There has been back & forth between DC Fire & EMS, & Metro as to the functionality of ventilation fans at L’Enfant Plaza on January 12, 2014.
Here’s the most likely explanation:
At 3:06 the power cable which transfers 750 volts from one third rail segment to another begins smoking and opens (or trips) the circuit breaker near 9th & Water Streets SW. This location is just prior to the Yellow line crossing under the Washington Channel, it’s also the location of both a ventilation shaft & emergency egress shaft, and it’s the location where the Yellow Line inbound track separates to cross under the Green Line to join up just south of L’Enfant Plaza.
Once the cables started smoking, at least two Yellow line trains traveling inbound toward Greenbelt passed through the area; now as most Metro passengers know when a train moves through a tunnel it pushes air ahead of it, and the tunnel just prior to the location has openings between both tracks to equalize airflow, therefore a train heading inbound could create a pressure wave that would push smoke on the outbound track back toward L’Enfant Plaza station.
Also after a train has passed there is a low pressure or vacuum created behind a train which would pull smoke behind it. So the combination of two Yellow Line trains passing the incident area would have moved smoked back toward L’Enfant Plaza while the vent fans which were activated at 3:16 would have in effect countered each other – Vent Fans pulling smoke to the south & train air currents pulling smoke to the north.
Also Metro continued to operate Blue, Orange, & silver Line trains through the lower level, and while not as strong the pressure and vacuum forces from these trains would have assisted in pulling the smoke into the station.
Had all train traffic been immediately halted the vent fans would have cleared the smoke quicker.
Recommendation to WMATA: 15-5 Upon notification of an open (tripped) circuit breaker anywhere on the mainline of metrorail operations, immediately suspend all rail traffic within the two nearest vent shafts/ tunnel portals, and activate exhaust fans, if a station is located between the two venting locations, contact all station managers to inspect the stations for any sign of smoke, also activate cameras to view toward the location of the tripped third rail, if smoke is observed immediately suspend ALL rail traffic including on different levels if at a transfer station and initiate bus bridges.. Dispatch either Metro Transit Police or have an offloaded train proceed through the area at a speed no greater than 15 MPH to inspect area for smoke and or fire.