CONCORD -- BART service east of the Pleasant Hill station on the Pittsburg/Bay Point line resumed Sunday morning.

Tracks damaged by a Friday derailment underwent a final inspection, and a test train rolled through the Concord station before it opened again for business at 8:07 a.m., BART spokesman Jim Allison said.

Work to fix the damaged tracks continued through the night, as workers repaired the straightaway track that is used by passenger trains, as well as cables within the track that were damaged, Allison said.

Allison said the derailment occurred when a 10-car, out-of-service train traveled over an "interlocking," a set of switches that can transfer trains from one track to another. The repair work to the interlocks is not yet finished.

"The interlocking is still damaged," he said. "But we can run trains without the interlocking, and we can go about repairing that when we're not in the middle of commercial service."

The cause for Friday's derailment remains under investigation. It happened at 6:20 p.m. and left the four derailed cars strewed across the eastbound and westbound tracks. The front of the train's lead car hung on the edge of the elevated rails.

"We still don't know whether it was the track that damaged the wheels, or the wheels that damaged the track," Allison said. The train operator was the only person on board and was not injured. The four cars that derailed were lifted off the tracks by a crane and eventually will be taken to the Hayward repair yards, Allison said.

Allison said repair work to the track used by noncommercial trains still needs repairs, and that will occur during the hours the train doesn't operate.

The derailment was the second in three years near the Concord station. On March 13, 2011, two cars of a 10-car train with 65 passengers headed for San Francisco International Airport left the tracks in a slow-speed derailment, causing a few minor injuries. That accident was later determined to be caused by uneven wear on a track section and flawed maintenance on a train wheel that slipped off the rails.

Contact Rick Hurd at 925-945-4789 and follow him at Twitter.com/3rdERH.