The incident was first reported about 8:40 p.m. A BART police commander says passengers on a train were pepper sprayed by the suspects.
It was unclear whether they made off with stolen property.
BART radio communications offered more information about the incident.
OPERATOR: "OK, hold on here. I'm in car 1856.
DISPATCH: "OK, you're in the car looking for the victim.
OPERATOR: "OK, I've got a male here. Those girls maced him. Female looks like they've been maced by those girls."
DISPATCH: "OK, they were assaulted and maced."
Police are looking for suspects who ran away after the attack.
No further details were immediately available.