There were between 230 and 250 passengers aboard the train, said Metro spokesman Jose Ubaldo, who described the 15 injuries as "minor."
The impact knocked the front car of the electric train off the track while on a run to Long Beach. The other cars remained on the track, which reopened about three hours after the crash.
"We had an out-of-service bus turn in front of the train," said Marc Littman, a spokesman for the Metropolitan Transportation Authority, which operates both the train and the bus. "We don't know who had the right of way."
The crash comes just a week after a Metrolink commuter train smashed into a freight train in the San Fernando Valley, about 30 miles northwest of downtown, killing 25 people and injuring more than 130.
The Metro Rail's Blue Line, which runs from downtown to Long Beach, started service in 1990 and is the oldest part of MTA's popular light rail system.