Amtrak spokesman Marc Magliari said the incident took place Tuesday about 4:30 p.m. about three miles west of the railroad's downtown Milwaukee station, while the train of three locomotives and 12 other cars was moving at a very low speed.

The eastbound daily trip of the Empire Builder between Chicago and Spokane, Wash., had taken place earlier Tuesday, and the train was expected to move as normal in both directions Wednesday, he said.

The cause of the derailment remained under investigation, Magliari said.