Waukesha Subdivision

Construction of the line started in 1882 by the Wisconsin Central Railroad and was completed in 1886.^[1][2] The railroad was reorganized from bankruptcy in 1897 and became the Wisconsin Central Railway.

The Minneapolis, St. Paul and Sault Ste. Marie Railroad (the original Soo Line) gained control in 1901 and leased the WC in 1902. The Soo Line operated the WC as its Chicago Division. After the 1961 Soo Line-WC-DSS&A merger that created the Soo Line Railroad, the railroad reduced the number of its operating divisions from five to three; the Schiller Park to Fond du Lac segment became the Soo Line's First Subdivision of the Eastern Division.

After the Soo Line acquired the Chicago, Milwaukee, St. Paul and Pacific Railroad (Milwaukee Road) in the bankruptcy auction, the line, among others, was placed in the Lake States Transportation Division in an attempt to cut costs. After the failure of this cost-cutting exercise, the LSTD was sold to Wisconsin Central Ltd. in 1987, which was then sold to Canadian National in 2001.^[3]

The Wisconsin and Southern Railroad has trackage rights over the line from Slinger to Waukesha, and freight cars are interchanged between the two railroad companies in Ackerville.^[4][5] Metra, Chicago's commuter rail service provider, also operates on this line as its North Central Service route (between Chicago Union Station and Franklin Park, Illinois, NCS trains use the Milwaukee District West Line, but do not stop at any of its stations, with the exception of the River Grove station). In Forest Park, Illinois at Madison Street, the subdivision connects with the B&OCT (CSX)'s Altenheim Subdivision and continues into Chicago. Also, Soo Line passenger trains historically used this line at Forest Park to access the Baltimore & Ohio Railroad's Grand Central Station (Chicago).

One of the obstacles faced by southbound trains is Byron Hill, which was a helper district when the line was operated by the Soo Line. Canadian National double-tracked that part of the line shortly after acquiring it.