Human error is to blame in a train derailment in Mankato this morning...That, according to authorities.It happened around 9:00 near the rail yard at Poplar and 'D' Street.Company officials with the Union Pacific say a couple cars jumped the tracks when a switch was changed while a train was in the middle of backing into another siding.Rail traffic is shut down at least through today.The rail cars were empty, and no one was injured.Union Pacific officials say the derailment will cost at least 15 thousand dollars to clean up before figuring in lost wages and business.