Crews were dispatched between the Lake Merritt and Fruitvale stations at about 7:50 a.m. after a train operator reported seeing smoke, according to BART officials.
Crews discovered a smoking cable that supplies power to the electric third rail, and immediately shut down one track between the two stations.
Service was delayed by more than 15 minutes in the Dublin/Pleasanton, Fremont, Richmond, Daly City and San Francisco directions while trains single-tracked between the two stations, and riders were advised to seek alternate transportation.
The problem was resolved around 11 a.m. and both tracks were open again between Lake Merritt and Fruitvale as of 11:15 a.m., BART officials said.
There are residual delays for some trains, but regular service was expected to resume around noontime, officials said.