The long, eastern corridor at the Powell Street BART station terminates at a metal "wall" at the 4th/Market portal. The crack between the metal panels reveals that the massive corridor continues for a long distance and curves to the right.
 
Along the center of the corridor, however, is a long, sad parade of luggage that goes off into wherever that mysterious corridor goes.
 
WTF?

UPDATE: @SFBART said the corridor is officially called the "Yerba Buena Tunnel", a pathway to Moscone Center that was never completed (not to be confused with the Yerba Buena Tunnel that connects the eastern and western spans of the Bay Bridge). Perhaps it will eventually form part of the "Central Subway" connection?

Still no word on the suitcases.

UPDATE 10 July 2009: @SFBART says it's for a canine training exercise. BART has a K-9 unit trained in explosives detection.