Muni spokesman Judson True said trains were again running through the tunnel as of about 7:15 p.m. and shuttles will provide supplemental service between the Castro and West Portal stations until the Muni system resumes its normal schedule.
The derailment happened at about 7:35 a.m. today when a set of wheels on the second car of an outbound L-Taraval train jumped about a foot off the track and hit a communications antenna, according to True.
No injuries were reported, and riders on the derailed train were taken out of the tunnel on another light rail vehicle.
Trains ran throughout the day between downtown and the Castro station, and between the West Portal station and the ends of the K, L and M lines. Buses shuttled riders between the Castro and West Portal stations.
About 250,000 riders were affected today, according to SFMTA executive director Nathaniel Ford. On average, about 700,000 people ride Muni every day.
Ford said the communications antenna was repaired quickly.
The train was re-railed and moved from the tunnel by about 5:15 p.m. and crews then ran tests on the tracks before light rail service resumed in the tunnel.
Several jacks were used to lift the 40-ton train back onto the track, Ford said, and the tight space complicated the effort.
"There's no room for error," he said before the train was finished being moved. "We want to make sure we're not endangering our workers."
Ford said the transportation agency wouldn't know what caused the derailment until the tracks and train can be inspected. The maintenance logs also need to be reviewed, and the driver will be interviewed.
In August 2006, a similar derailment occurred in the same area, Ford said.
He said track repairs were done after that derailment but don't appear to be needed now.