Vegar Halveg, a local worker at the port, said he heard a loud screech and saw several empty cargo train cars speed past.
"They were going much faster than they usually do, so we realised something was wrong," he told the Associated Press,
"The last car was practically airborne," he said.
Several train cars also ended up in the water after crashing at the port.
Dozens of ambulances, several helicopters and a coast guard boat, along with other rescue workers, were sent to the scene, local media said.