Watertown Subdivision

west to Tomah Subdivision 178.2 Portage spur, formerly to Stevens Point, Horicon 177.0 to Madison (M&P Subdivision) 176.0 Pacific 169.0 Wyocena 164.3 Rio 159.3 Doylestown 153.6 Fall River 150.0 Columbus 146.2 Astico 140.8 Reeseville to Madison (Wisconsin and Southern Railroad) Union Pacific Railroad Clyman Subdivision 131.2 Watertown 123.6 Ixonia 117.9 Oconomowoc 115.9 Okauchee Lake 113.0 Nashotah 110.1 Hartland 104.2 Pewaukee CN Waukesha Subdivision flyover (proposed) 102.1 Duplainville Canadian National Railway Waukesha Subdivision 99.4 Brookfield 95.1 Elm Grove Union Pacific Railroad 90.6 Wauwatosa to North Milwaukee to Muskego Yard 85.7 Milwaukee Intermodal Station south to C&M Subdivision

The Watertown Subdivision or Watertown Sub is a railway line in Wisconsin operated by Canadian Pacific Railway (CP) through its primary United States subsidiary, the Soo Line Railroad. It meets CP's Tomah Subdivision in the west in Portage and runs to Milwaukee in the east where it meets the C&M Subdivision. The Tomah Subdivision had previously been operated by the Chicago, Milwaukee, St. Paul and Pacific Railroad (CMStP&P), though Canadian Pacific took it over when the Milwaukee Road folded.

This line contains the oldest stretch of track in Wisconsin, constructed by an early incarnation of the CMStP&P, the Milwaukee and Mississippi Railroad. Track was laid between Milwaukee and Brookfield in 1850. The initial line then turned southwest from Brookfield and went toward Waukesha. The first passenger train ran between Milwaukee and Waukesha on February 25, 1851. The next segment of the present subdivision was finished between Brookfield and Watertown in 1855, and continued partway to Portage in 1857. This line didn't reach its present western end at Portage until 1864, though an alternate route farther north that ran through Iron Ridge and Horicon, Wisconsin, had reached the city in 1856.

Farther to the west, today's Tomah Subdivision was quickly built from Portage to La Crosse, Wisconsin, in 1857 and 1858. The line finally crossed the Mississippi River into neighboring Minnesota in 1866. This was the second line of the Milwaukee Road to reach the Mississippi. The original rail line to Waukesha had been expanded through Milton and Madison, and reached Prairie du Chien in 1867.

In 1935, the Milwaukee Road introduced the Hiawatha passenger train which ran at high speed between Saint Paul, Minnesota, and Chicago. It was one of the fastest passenger trains in the world at the time. When Amtrak took over passenger service in the United States in 1971, the former Great Northern Empire Builder was redirected to run along the Hiawatha's route between Chicago and Minneapolis–Saint Paul, Minnesota. The Empire Builder still runs along this track, and makes stops in Portage, Columbus, and Milwaukee. The Amtrak Hiawatha Service, which now runs between Milwaukee and Chicago, is expected to be extended to Madison and will add stops in Watertown, Brookfield, and possibly another city. Initial plans to include a stop in Oconomowoc were dropped.