The Chicago and Milwaukee Subdivision (commonly referred to as the C&M Subdivision or C&M Sub) is a railway line running between Chicago, Illinois and Milwaukee, Wisconsin. It is operated by the Canadian Pacific Railway through its primary United States subsidiary, the Soo Line Railroad. This had previously been owned by the Chicago, Milwaukee, St. Paul and Pacific Railroad, commonly known as the Milwaukee Road. It carries Amtrak's Empire Builder and Hiawatha Service, as well as commuter trains operated on part of Metra's Milwaukee District/North Line into Chicago. The Union Pacific Railroad operates their Milwaukee Subdivision, a former Chicago and North Western line, parallel to this route just to the east.
This line once carried the Milwaukee Road's fast Hiawatha passenger trains, including trains that regularly exceeded 100 miles per hour (160 km/h). For several years in the 1940s and early 1950s, trains were scheduled to run along this stretch in 75 minutes or less. While stricter regulations eventually reduced the speed limit to 79 miles per hour (127 km/h) along this stretch, plans are in place as of 2010 to improve the line to support speeds up to 110 miles per hour (180 km/h)—modestly faster than historic trains that used the route.