The derailment of an outbound light rail vehicle, one headed away from downtown San Francisco, was reported at about 7 a.m., Muni spokesman Paul Rose.
The derailment has caused delays throughout the system, Rose said.
"There's slower subway service" than usual, "but we're moving," he said.
The derailed light rail vehicle remained stuck near the Castro Street station as of 8:40 a.m., and crews are trying to determine the cause of the derailment, according to Rose.
Bus shuttles are taking passengers between the Castro Street and West Portal stations while crews work to restore normal service, he said.