HOUSTON (August 15, 2012)—No one was hurt early Wednesday in the derailment of seven empty cars at a Union Pacific Railroad yard in Houston.

Crews were working Wednesday to clear the accident area.

Union Pacific spokeswoman Raquel Espinoza said at least three of the freight cars overturned.

Authorities are trying to determine what caused the derailment as a train was being assembled.

Union Pacific is based in Omaha, Neb.