Update, 8:55 a.m.: BART says that crews have arrived at Embarcadero Station and are "looking into the fire or smoke issue on the trackway" at the station. The agency said via a recorded message that the exact nature of the problem is unclear. At first, BART said a small trackside fire had occurred at or near the station; now it says the issue might be smoke, and it's trying to determine the source.

The agency said fire crews were standing by if needed. It also reported it's seeking "mutual aid" from Muni to help move passengers in downtown San Francisco.