ORINDA -- A smoldering fire underneath a BART train that forced passengers to be evacuated Wednesday appears to have been caused by a piece of propulsion equipment that overheated below the tracks, a BART spokeswoman said Thursday.

BART reopened tracks at the station in both directions around 6:30 a.m. Thursday, shortly after the affected train was moved out of the station, spokeswoman Alicia Trost said. The train went east to the Concord maintenance yard, and other trains moving along the line were expected to be delayed up to 15 to 20 minutes as the system recovered.

The chaos began just after 6 p.m. Wednesday, when smoke coming from under a Pittsburg/Bay Point-bound train was seen. The fire smoldered for a couple of hours and flared up again after 8 p.m., Moraga-Orinda Fire spokesman Stephen Healy said Wednesday.

Trost said a motor reactor, a 1,200-pound piece of equipment used in the propulsion process, overheated. Fire crews needed to blowtorch through the tracks to reach the reactor and put out the blaze. Trost said a more thorough investigation into what happened will take three to four days.

Trains moved through the Orinda station without stopping until 8 p.m. Wednesday, when passengers were allowed to board and disembark trains at the southern end of the platform, Trost said. The entire platform was reopened around 9 p.m., although the eastbound track remained blocked and trains moving in both directions used a single track to get through.

The electrical third rail was not damaged in the fire, Trost said.

Camino Pablo, the street that passes underneath the Orinda station, was closed in both directions until about 10:30 p.m.

Some passengers said no announcement was made regarding the emergency, but Trost said Wednesday that an announcement was sent on the agency's public address system. She said the agency would investigate why the announcement may have been difficult to hear.

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