SWAT teams from the FBI, San Francisco and BART police ran through different emergency scenarios -- passengers being taken hostage, an armed assailant, explosives on a train.
"Roughly every four months, we do what we call 'tubular training;' it's vehicles like planes, trains buses," FBI spokesperson Tom Ravenelle said.
Wednesday's exercise on the trains was timely; Najibullah Zazi, the Afghan native with ties to al-Qaida, just pleaded guilty to federal charges that he and others made explosives to blow up trains on the New York subway system.
At the Millbrae BART station, two platforms were closed off and two trains were out of service for the all-day exercises.
"What we learned today, which will be different, is how to approach and how to enter and how to deal with people once we get in," Ravenelle said.
Trains, buses and planes pose different challenges for SWAT teams, which are more used to tactical assaults on houses and other facilities.
For one thing, the suspects can blend with the passengers.
"So you have to treat everybody as a potential subject until you can sort it out after the danger is taken away," Ravenelle said.
The sight of heavily armed law enforcement agents took many BART passengers by surprise. They thought, surely, something was going down.
"As I'm walking by I actually see them pointing guns inside the train so I thought, 'Oh man, what is going on,'" BART passenger Xavier Alexander said.
"That reminded me of those action flicks you know, adventure and everything," Saurabh Kumar said.
Even those passengers who were scared by the realistic training said the fact that they were training was comforting.
"As an American citizen, it makes you feel a lot safer to know that they actually practice these things," BART passenger Joe Jackson said.
Said one passenger, the worst thing is to be unprepared for an attack.