A car that carried only women passengers on the electric train overturned and the truck caught fire, the Jakarta Globe reported.
"Seven people have died and 20 are injured, the train driver was still trapped in the front carriage," the report quoted National Police spokesman Rikwanto as saying.
Commuter Line (KCJ), a subsidiary of state-owned train company Kereta Api Indonesia, said rescue operations were still under way.
One passenger in the rear part of the train told the Jakarta Globe brakes were applied before the train reached the crossing.
"After that, the train collapsed to the right side and I heard the explosion," he said. "The access to the first car was blocked and I couldn't go in there."
He said passengers trapped in the overturned car tried to break windows to get out.
A spokesman for Pertamina said the tanker belonged to the state-owned energy company, the report said.
"Regarding the cause of the accident, we are still waiting the official investigation from the police," Pertamina official Ali Mundakir said.
"The standard operating procedure for gas trucks is very tight and this will be a part of an investigation that we will conduct," he said.
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