Investigators believe a defect with a track switch probably caused a Green Line train with 200 passengers aboard to derail Friday morning, MBTA spokesman Joe Pesaturo said.
Pesaturo said workers would make permanent repairs at the end of regular service Friday night.
The trolley, heading west out of Government Center, derailed at low speed shortly before it entered Park Street Station around 9 a.m. Friday, the MBTA said.
No one was injured.
Full Green Line service resumed shortly before 1 p.m., with "severe residual delays" persisting, the MBTA said.
Jeff Durney was riding the train behind the one that derailed, he said. When riders got the news that the train was stopping between Government Center and Park Street, his fellow commuters were annoyed, he said. But they became calm when they were told to evacuate.
Riders got out in the tunnel between the stations, and walked back to Government Center.
"Interesting morning . . . It's kind of weird, kind of different," he said. "I've never walked in a tunnel like that before."
Durney said the T was "actually really great about it," and helped people along the tracks.
One woman had a baby in her stroller, so MBTA workers carried the infant over the tracks, said Shanan Kumar, who was on the same train as Durney.
"Everyone on my train was OK," she said. "They said it was safe but I was obviously a little worried about stepping on the wrong thing."
J.D. Capelouto can be reached at firstname.lastname@example.org. Follow him on Twitter @jdcapelouto.