Regular subway service resumed on the Red Line between the Ashmont and JFK/UMass stations in Dorchester around 9:02 a.m., marking the end of shuttle buses that were deployed around 5:30 a.m. when a train derailed in the Ashmont train yard.
The T warned that there could be residual delays, and that the shuttle buses are steadily being taken out of service as riders return to the rails.
The shuttle operated in both directions on Dorchester Avenue during the rush hour, picking up passengers near the Shawmut, Fields Corner, and Savin Hill stops before reaching the JFK/UMass station for transfer onto trains.
The Braintree line was not impacted.
The T issued its first alert around 5:30 a.m. T spokesman Joseph Pesaturo wrote at 7:34 a.m. that the derailed car has been returned to the tracks, but workers first had to repair the track and third rail before service resumed.
Service resumption was tweeted around 9:02 a.m.
Pesaturo said the train was not in service when the derailment occurred, so there were no passengers on board.
“One car of a six car train derailed,” Pesaturo wrote in an e-mail. “The cause is under investigation.”