Muni trains were halted during the Monday evening commute because of a problem with the system's signals, a transit spokesman said.
Underground Metro service was stopped because of electrical problems caused by a transformer that blew near the Van Ness Avenue station about 5:45 p.m., said Muni spokesman Paul Rose. Stranded passengers were being taken by bus shuttles between the Embarcadero and West Portal stations.
Crews were working on restoring service, but Rose could not say when trains would be running again.
In a separate incident, a woman in a wheelchair was hospitalized with life-threatening injuries after she was hit by an F-Market streetcar about 5:30 p.m. near Church and Market streets, said police spokesman Officer Carlos Manfredi. Because the woman was trapped underneath the train, special equipment had to be used to lift it so she could be freed, Manfredi said.
Investigators were interviewing witnesses and would try to determine if the incident was recorded on surveillance video.
Rose said the two incidents are unrelated.
This is the second time in two days that Muni has experienced major delays and problems. On Sunday morning, Metro service was halted for hours due to flooding in the Castro Street station. Crews worked for five hours to pump water from the tracks. More than an inch of rain fell in downtown San Francisco between 4 and 11 a.m. Sunday.