The Sioux was a named passenger train of the Milwaukee Road that operated between Chicago, Madison, Wisconsin, and Rapid City, South Dakota, via Prairie du Chien, Wisconsin and northern Iowa.

[3] West of Madison it also operated as a mail train, making frequent stops.

On October 1, 1951, the train was cut back to a Chicago to Canton, South Dakota, service with prepaid taxi connections to nearby Sioux Falls.

In the 1950s, the Sioux itinerary between Chicago's Union Station and Madison, Wisconsin was limited stops between Chicago and Walworth; and service in that latter territory was handled by suburban Milwaukee Road trains.

[5] The train ceased operation on May 1, 1971, when Amtrak assumed responsibility for providing a national rail service.