Those tracks were the Chicago, Milwaukee & St. Paul, an ordinary railroad line built 1885-89 between Chicago and Wilmette. The corporate history is rather complicated, but the part from downtown north to Wilson was used for ordinary freight service to North Side industry during most of the 20th century. As the Northwestern Elevated Railroad (today’s CTA Red Line) extended service north of Wilson circa 1907, they took over the tracks from there north, though arrangements to serve freight customers north of Wilson continued even into the 1970s.
In the 20th century, the railroad tracks were a freight spur that ran north from the area of Union Station, crossed the Chicago River at a sharp angle near Kinzie Street, then ran down the middle of Kingsbury Street, then north in the middle of Lakewood Avenue before curving a little eastward just south of Addison. They served coal yards, the brickyard where Wrigley Field was built, lumberyards, and other small industrial users. In recent decades the connection to downtown was severed, and the tracks were abandoned north of Diversey sometime in the 1980s. The last customer was the Peerless Confectionery factory on Diversey.